How can I rent a mobile WiFi hotspot during my five-week trip to the UK?

This answer is some potentially useful information from the OP about my experience.

When I arrived in Heathrow I found there was an EE shop, which had a special deal on exactly this, a 3G/4G-to-WiFi pocket device. To use it you have to buy both the device and a top-upable one month SIM card - I think this was about £60 in total. However, they were out of stock. (They told me their counterpart shop in a different terminal had it in stock, so I went there but they were out of stock too and for some reason were quite rude about it. So basically don't trust these guys, as they are absurdly disorganised.)

So in the end I got such a device from Curries, a high-street electronics store. I actually got a contact of mine in the UK to sign up for a monthly contract and then cancel it on the day I left, which cost the same but meant there was a larger data cap, and it worked fine. Because of this, I'm not sure whether it's possible to buy the one month version without proving UK residency - if anyone can answer that it would be helpful for future visitors.

I have used a mobile dongle in the UK before while waiting for a phone line to be connected in my new house - it was alright, but signal can be variable depending on what area you're in - most cities are OK but can still have some blind spots on certain networks.

I bought a dongle for around £15 - the main cost will be the bandwidth. Based on my experience, expect it to cost you around £10 per gigabyte. As @Cmaster mentioned, Carphone Warehouse is probably your best bet, they're everywhere, and are quite helpful in terms of getting you the info you need and finding the right option for you.

There's a good comparison here:

http://www.uswitch.com/broadband/compare/pay_as_you_go_mobile_broadband/?gclid=CJrBpo7J4sUCFfMZtAod6lMAZg

If you think you'll be using 12gb of data then you can buy this upfront in a package for £134.99 including the cost of the dongle and SIM.

Or here: http://www.vodafone.co.uk/shop/tablets-and-dongles/dongles-and-mobile-wi-fi/index.htm

Where you can buy a dongle for £15 (including 1gb data), and then buy data at £10 per gb, or £20 for 3gb.

I am not aware of short term rental of either USB 3G/4G dongles in the United Kingdom, and have not seen these advertised either in domestic mobile phone shops or when passing through British airports - it should be noted though that this travel has not often been for business purposes, and I already have unlimited data and wireless tethering available within Britain and some European companies which obviously means I wouldn't have been actively seeking such offers out.

There is near market saturation of smartphone ownership in Britain (93% ownership by 18-24 year olds and 81% in the 25-34 year old age group in a survey from 2013) and as such the market is highly competitive - although not as competitive as in some other European countries such as Estonia (based on month long stay by my wife last year). Several "virtual" networks (who do not own their own network infrastructure but buy access wholesale from one of the primary networks) are competing in the United Kingdom domestic which is helping keep prices down.

Smartphone and tablet usage is extremely high and there has been a push by networks to sell separate "mi-fi" hotspots for use with tablets and laptops. These are available both with pre-pay bundles or on monthly rolling contracts.

The UK network "Three" offer a monthly rolling contract on a mi-fi portable hotspot at £29.99 GBP (approximately 42.22 Eur, 5698 Japanese Yen) up front and £15.99 GBP (22.51E / 3036Y) per month for 10GB data. It would be possible to cancel the contract giving a month's notice a week in to your visit - which should mean the total is only in the region of £50GBP. Given the price of 24 hour passes at wireless hotspots at premium locations such as airports in the United Kingdom (most train stations and many pubs, cafés and restaurants will offer free wireless access) I'd imagine you'd struggle to find wireless hotspot rental much cheaper - if indeed the option is available. The data allowance is generous, and you'd own the device (which is sold unlocked) at the end of the contract. This device isn't 4G capable (although your contract would be as Three don't charge extra for 4G data usage), but has a generous data allowance and is theoretically capable of 21.6Mbps throughput according to the manufacturer. If 4G speed is more important than data allowance then there is a 4G option with half the data allowance available at the same monthly price - and slightly more upfront for the device, which has better specifications. I'm not an expert but there appears to be some frequency overlap with Japanese frequencies so it is possible that the devices might work back home in Japan - but even if not would be worth holding on to for future trips to Europe.

The UK's largest mobile provider EE has an even more generous 15 GB allowance 4G 30 day rolling contract mobile wireless hotspot available for £20GBP per month (28.16E / 3799Y) and £39.99 GBP (56.29E / 7596Y) upfront. The device may be locked though, so be aware and ask before you buy.

If wireless isn't important to you and you only plan to use the internet with a laptop then it would be cheaper in terms of upfront cost to buy a USB modem/ dongle (it may be worth noting that these are probably the first thing most people will think of given the word "dongle") which is available in both 3G and 4G variants.

Depending on your residence arrangements whilst you're staying in the UK, it may not be possible to obtain even a thirty day contract if you don't have an address to give the network. The absence of any history of credit in the United Kingdom might be a factor, but the risk to the network is relatively low with the device pre-paid and a low monthly cost are in your favour here - you're not asking for a new iPhone or similarly expensive piece of hardware. Another limiting factor might be a possible requirement for a UK bank account to pay the bill - although it might be possible to pay using a credit and/or debit card over the phone.

As a backup, should any of these factors make it difficult to get a contract set up, you can obtain the same hardware as a pre-paid bundle costing £59.99 GBP (84.44E / 11394Y) for the 3G hardware and £69.99 GBP (98.52E / 13292Y) for the 4G. These both only come with 3GB data allowance, as it sounds like you need more you might need to bump up to the 12 month pre-paid bundle at £109.99 GBP (154.85E / 20891Y) for 3G or (168.93E / 22790Y) for 4G.

It might be worth contacting Three in advance (they have online chat or email) to see what you can do or bring with you from Japan to help obtain the thirty day contract as your data needs in the UK seem significant and the contract offers much more data at less cost. Their website does specifically mention contacting them by phone if you don't have UK address history, so there is reason for hope.

I can't tell you about Heathrow airport arrivals, however in pretty much all towns and cities (and in several "out of town" sites as well), you will find the general purpose mobile shop Carphone Warehouse, and shops for the four major mobile networks: EE, Vodaphone, O2 and Three. Any of these will happily sell you a dongle and a pay as you go/one month contract sim. If your usage really is >150mb every single day, be warned that it is going to be expensive (also, is your email full of videos or something?). Be cautious of "unlimited" data packages, they often have restrictive fair use paackages.

For example, Carphone Warehouse's mobile broadband page (I am sure their in-store reps would be better able to explain exactly what would be the best deal) http://www.carphonewarehouse.com/broadband/mobile-broadband . As you can see, the costs involved in acquiring the hardware are low enough to make hiring probably uncesessary.

It's also worth noting that you are likley to get wifi access in your hotels (if you are staying in them) and there are free hotspots available in many cafes. So you could do your email checking etc at these places, and save traffic on your mobile broadband for more urgent needs.