IDNet [as you mentioned] or Zen's active 8000 package
http://www.zen.co.uk/Broadband/S_Business.aspx?page=518
will fit your requirements perfectly.
Check out Newnet
http://www.newnet.co.uk/broadband/home.php
as well, 3 quid cheaper with 25GB usage included but uploads are counted in the total unlike IDNet's 30GB and Zen's 20GB where only downloaded data counts [unlimited upload on both].
As for UKFSN [they count uploads in their usage btw], they're nice and fast especially if you use your connection between the hours of 8am to 10pm monday to friday and can live with a [30GB uploads included cap. [on the 20 quid max home package]
After 10pm [and all weekend] things slow down [usually] because of the much larger off-peak allowances kicking in, and everyone starts firing off torrents ect and the pipes get full. But to stop them getting saturated and affecting ping times and causing packet loss a tool slows your connection to 2Mb, but ONLY if the pipes get full. If you only can sync at 2Mb speeds or slower then it won't make much difference.
entanet/ukfsn