You've got two possibilities:
a) Cable, if you've got cable TV (NTL or TeleWest) or if it's available to you
b) ADSL if you're within 5 km of a BT exchange that's ADSL enabled
First thing to do ... find out which (if either) is available for you; you can put in your postcode on the relevant sites and they will tell you (they may say "subject to line test").
I did some tests about 6 months ago. Download of 1 Mbyte via a modem - about 4 minutes. Via 512k ADSL - about 30 seconds. Via 512k Cable Modem - about 20 seconds (the difference being due to line sharing and contension with ADSL).
Tiscali offer a SLOWER ADSL service (150k) as well as the standard service and they're adverising it as high speed, and there are a number of higher speed services. We use a 2Mb cable service here and can download a Megabyte in 4 seconds ... but then we need it for work, and it costs us a bit more.
I wrote an article on this in our company newsletter - downloadable from
http://www.wellho.net/downloads/index.html and you want the Autumn 2003 edition, page 9. We have a further article on wireless in the Spring 2004 issue.
P.S. If neither of the above is available, you may want to look at ISDN - though I would NOT recommend it for new installations normally - or Satellite Broadband if you need it in spite of the high cost and hassle. Please feel free to p.m. or email me.