I think mentioning TalkTalk should go through the swear filter. Yess LLU can be better if there is a good ISP running it, unfortunately there doesn't seem to be many good isp's on most llu's anymore, perhaps things might change.
Actually all LLU services are very good in terms of the actual connection, its just the customer support quality which varies vastly from ISP to ISP. ANY LLU isp is about a trillion times better than using BT ipstream or even their 21CN service...on the vast majority of BT exchanges up and down the country, slowdowns are common at peak times on BT ipstream services.....that hardly ever happens on LLU services.
OP, if you have a good knowledge about how broadband works and don't use P2P and don't mind using talktalk's web support portal, then go for talktalk. I also used to be a talktalk hater like millions but was brave enough to try them out and haven't regretted it one bit
If you don't want to touch talktalk but they are the only LLU provider at your exchange, then go for a talktalk reseller such as Xilo or Vivaciti...they'll be slightly more expensive but they'll offer superior customer support over phone, no P2P throttling though they do have downlimits in place.
If you also have C&W LLU then again consider using Xilo or Vivaciti or ADSL24. Aagain unthrottled service, superb customer services with no download services.
Also you wouldn't go too far wrong with Sky LLU
TalkTalk LLU rocks
IDNET ipstream sucks