Im no expert andys, but I run an online development company so know a little.
As JCJC says, distance from exchange is going to be a big (BIG!) factor, but a noisey line or even with bad latency isnt only going to affect upload speeds - you would see a degradation in both d/l and u/l if this was the case I think.
Most likely I would suspect is your ISP contention ratio - in my experience the biggest single reason for poor Internet performace after distance from exchange.
My advice would be to first check out the "super" broadband providers as they will give the best speed:
Virgin, Be (my mate is with them and says very good indeed) and now BT Infinity (in my area now and are "estimating" my speeds at 18Mb download and 2Mb upload.... nice (if true!!)
After that, my own decision was to go with O2.
Very good speeds, cheaper than BT, and (the deal maker...) if you are an O2 customer (like have a pay-as-you-go mobile) you get the broadband for half price AND (!) if you then register with QuidCo, at the moment you get £100 back which is pretty much what you will pay for a year... so its a year of decent speed broadband for free
...which is nice :P
Oh, for comparison, O2 are now supplying me with a steady 6.5Mb download and 1Mb upload. It aint going to win any speed awards, but much better than what I was getting with BT - and its nearly free.
Hope it helps mate.