That is actually not so bad. 6pm to 11pm is only 5 hours out of 24

Many users have a lot worse terms and conditions

I don't live in the UK myself so don't know which service is the best there, but I will pass this on to someone who does later today

I've been using ukonline for the pass 6 months and have downloaded quite a lot (as I had to reformat my hard disk and didnt save any personal files!)
They've been quite good and so far, there is no download or time limit, the price at the moment is resonable too.
If you go for an ISP, go for one which offers an umlimited download rather than one which limits your download to say 2GB per month.