BT Broadband as with other service providers slow the service down between 6pm and midnight, or there would not be enough bandwidth to go around.
Try a speedtest at
http://www.grc.com/x/ne.dll?rh1dkyd2
Not entirely correct

Some ISPs do nothing - Zen Internet being a prime example. You get whatever the product can deliver (if thats a contended service, obviously subject to contention).
Some ISPs do mild shaping - BT are an example, usually slowing down non web traffic, but otherwise, full capabilities of the product.
Some, Plusnet being the first to really use it hard, shape some of their products to make it almost unusable.
Wholesale charging structures, introduced to cut down the unsustainable p2p/nntp traffic at a wholesale level, have meant some of the oversubscribed ISPs have to shape heavier than others.
Most DSL and all cable services are contended, and if you have a load of heavy users shared with you, you may suffer whatever you have.