Dunno what the Beeb recommend, but I would have thought between 1.5 and 2 Mbps was adequate. You can expect varying speeds depending on the time of day, and the contention ratio in your area, but I would have though a drop from around 4 Mps to less than 1, would indicate some sort of problem.
You might want to do a ping test as well:
http://www.pingtest.net/
"what bandwidth my isp is supposed to provide me with? "
Well only you can answer that - what does it say in your contract?