Something else to watch for is whether your ISP account has set limits on how much data traffic is allowed (some of the cheapest offer 1 GB a month, 3 GB a month and so on).
Not used iPlayer for much in the way of video, but if you also use Channel 4's "4oD" (4 on demand) or similar service for five.tv then some programmes are quite big. Grand Designs and other 30-60 minute shows can be 250 to 400 MB (which obviously will take some time to download, but also eats up ISP allowance.)
(I have a 5 GB limit each month, but may switch to opal.co.uk which only charges a fiver a month in the first year, tenner in the second, with a 'fair use' policy but no fixed download limit. It's part of TalkTalk/ Carphone Warehouse, and unfortunately not on all exchanges.)
As for slow speed, there's a site about getting Faster Broadband and a petition on the number 10 website to do with connecting the BT cabinets (green boxes on the pavements - probably near a village post office, or where PO used to be!) linked up with fibre to exchange, which would allow 15 to 40 Mbps for people even if their exchange is 3-4 miles away.
Thanks for the big screen link, nannynick - something geeky to check out