First of all you need to realise that there are really two key speeds.
1: Sync speed – the speed that your router/modem synchronises with the DSLAM in the exchange.
2: For any BT supplied ADSL services (in most cases ISP’s utilise a resold BT service, unless you are with an ISP who utilises LLU and has their DSLAM co-located within the BT exchange) an IP Profile. This profile based on 20CN exchanges is usually based on your lowest sync speed over a period of time, about 3 days usually. On 21CN exchanges it is 88.2% of your sync speed.
In theory then your ISP should be giving you a throughput based on your IP profile, however if their backhaul from the DSLAM or their own network is congested (I’m including their Transit/Peering within this) you will be seeing speedtest results lower than your IP profile.
So once the line is stable, note your sync speed, then go to http://test.speedtester.bt.com and follow the instructions there. This will give you your IP profile and what your line is capable of to the Exchange.
Then do some speed tests to the likes of speedtest.net etc and see what you get there. If there is a big difference between the two then its likely to be congestion on the ISP’s network. Getting them to then admit there is depending on ISP can be quite a challenge, I’ve had quite some discussions with friends/family ISP’s in the past and the advice I offer is don’t buy the cheapest…