T-mobile seems by far to have the best fair use policy in the UK. the only provider that won't charge you per Mb after you run the limit.
However it seems that to get full hsdpa speeds, it is necessary to get either a "Mobile Broadband Plus" or "Mobile Broadband Extra" package. It's my understanding that the Web'n'Walk or Internet booster will only get you a speed of about 380Kb/s.
The problem is, that about a week ago, the "Mobile Broadband" options were removed from the sim contracts. here you can see that it advertises "Mobile Broadband Plus: £12.50/month or £10.00/month for duration of plan, 3GB fair use." However that option isn't in the drop-down menu. The only way now to get a Mobile Broadband package is here.
Would it be possible to take the sim out of one of these usb modems, and use it in a phone? And would it be possible to get the phone a number, if that were to work?

Thanks.

I am in the US, but i had a pcmcia card from t-mobile. It had to have its own phone number to work. I could take the sim card out and use it in my n95 for internet just fine. But those modems typically dont have voice plans, so calls might be 10 cents/min.

I wonder if there is any way to add on a voice plan and a few texts. I guess with the N900, skyping out to people would be easy, although how much would the battery run down, having a skype connection on all the time for incoming calls?

i use skype and google voice on my n900. not a big battery hit. but im wondering why go to the effort? my current tmobile plan is $51/month for 500 min and unlimited data. i thought that was pretty good. are you trying for even cheaper?

oh i prob dont have the download speeds you are after

Sadly, google voice isn't available in the UK. And "Unlimited Data" is actually pretty limited in the UK. For example Vodafone's definition of "unlimited" is "Limited to 500MB a month".

And to take full advantage of hsdpa, you need to get the 3gb, or 5gb tmobile data plan, which at the moment, you can't get with minutes or texts.