Read the T&C's
very carefully - many of these companies that advertise 'unlimited' broadband, Virgin, Tiscali, Wanadoo etc. do nothing of the sort.
What they do have is a 'fair usage policy' which limits your downloads each month. Also some of these deals block access to P2P which is a bit of a killer
If you want fully unlimited/unmetered then expect to pay around £30+
Check out
adslguide and
ispreview for details on each deal.