I've got Plusnet at my London house, and it got *very* flakey - with frequent drop offs and periods where it just was totally unusable. Its stabalized a bit over the last few weeks but im far from impressed.
Plusnet also seem to have issues with torrents (I use torents for legitmate use I should add), either from them firewalling, or network problems (its hard to tell when you have limited net access

). And their support site didnt say anything about specifically blocking it.
Portsmouth house is currently using NTL - and I must say, I've never had an issue with NTL down here (was down for 2 hours last week, but they sent a letter out about a week beforehand stating maintainance, so I just went out for a while

).