It's worth checking your connection speed here:
http://www.bbc.co.uk/iplayer/diagnostics
(As well as providing raw figures that test also tells you whether your speed is good enough for different levels of iPlayer service).
Also look on iPlayer pages for an option to switch to a low bandwidth version (which doesn't require such high speeds as the standard version).
Then hassle BT and tell them that you're fed up with the 'ADSL Max' service that you're getting (because that's the default service that BT provides) and you want an upgrade to 'ADSL2+' (which will roughly double your speed).