Depends a bit on how much data you are likely to use.
In the past I've had a contract (happy with it, but unwilling to commit to a further 2 years, as the deal was half price when taken out in Sept 2008). Contract was for 15 GB/month for 7.50 (went to 8 quid with VAT rise).
On Ebay, there are sellers offering SIMs already credited with 3 GB for up to 3 months, for about 13 pounds (once was a time when they were sold for 8 or 9 quid!)... you have use of the internet for 3 Months use or 3 GB (whichever comes first). Ideal if you have busy weeks and slow weeks, but know you will use up that amount, in anything up to 3 months, without being charged the earth with only 30 days validity.
Also worth getting, if you did use Three (and check their coverage in your area on the www.Three.co.uk site) is that Sambamobile.com offers a cheap SIM or SIM + dongle deal (and you increase credit by watching adverts... you get 3.5 MB added on when you watch an advert. If there's anyone you know with an 'unlimited' broadband connection, it would only take a few minutes to credit you with say 50 MB (it is not time limited) to keep as an emergency... for example, to get onto Ebay to buy another Three SIM.
So if you are able to get a signal from Three, I'd suggest getting one of their PAYG packages (perhaps check HotUKdeals.com to see if any store like Argos has a deal on for Three broadband) - also see if Quidco or TopCashBack has a deal... Just checked -
TopCashBack.co.uk offers 25.75 cashback on Mobile Broadband 1 month rolling contract, or 13.17 against PAYG broadband. Cashback can take some months and not guaranteed, sorry. With cashback sites, you login on cashback site, then go to store and buy product, and a 'cookie' from the cashback site is detected to 'track' the sale and pass commission back to the cashback site. I think TCB site charges a small annual fee (6 quid?) so it would come out of the saving you make. Don't think Quidco charges a fee.
*> Three 1 month contract costs approx 40 quid (so you'd pay for the data not the dongle...
*>Three