HB - I'd second the comments above, about using a dongle. It depends how important the internet is and how long you expect to be away.... 20-30 quid for using the internet for a couple of days would be 'expensive' in my view and I'd do without, but if I was expecting business e-mail and away with someone for a week, it may be worthwhile...
One thing to check for is whether you can get a deal including some amount of web traffic. I looked at Asda's online site because I have seen dongles at about 20 quid in Asda Living... There's one from T-Mobile which costs 20 quid (and there may be a mandatory 10 pound top-up), while one from Vodafone (item 871-4441 on Asda Direct, which can be available with no extra charge at your local store) for a quid more includes 2 GB of data. O2 and T-Mob charge 2 quid a day (other options available) while the Vodafone one has 2 GB valid for 30 days as far as I can see.
I am using a Three dongle (have been since October 2008) and can say that if you have coverage, it might be worth looking for one from a Three store, as there are people selling SIMs preloaded with 3 GB/3 months (expires when you hit either limit) for under 12 quid, on Ebay. (Must admit I paid around 8, but even at 12 pounds it is better value than a 15 pound top-up giving only 2 GB which is what is offered by Vodafone).
Watch out for "unlimited" as some of the mobile networks suggest 50 MB to 250 MB is adequate, and it is easy to exceed in my view (I use 15 GB a month, and at least 100 GB on my landline internet service).