Depending on how long you are in uk it might be worth looking at the 3 skypephone S2. £60 PAYG or on 6 month contract £15 (phone free & 300mins/month). This can be used as a 3G HSDPA dongle for an extra £5 a month (1G allowance "existing customer discount" phone counts as existing customer) You do have to select the right options though
You can also use the net on the phone, albeit with a limited browser.
T-mobile may say you cannot use a phone as a modem on their cheaper deals, but you can. I use my 3G phone as a modem with my laptop and get 480kbps connection with the cheapest of T-mobile deals.
TudorSailor
If it's cheap and easy PAYG that you require then look no further than ASDA mobile. It's only 8p/min for calls and 20p per MB. You can pick up a SIM card in the store for 50p.
ASDA mobile runs on the Vodafone network so excellent coverage.
Some providers eg Tesco don't allow GPRS modem usage, only WAP browsing. But ASDA is OK with a laptop.
Interestingly Vodafone don't have a cheap option with their PAYG plans - you can't buy the £5/month data pack unless you have a pay monthly plan.