The words up to are being abused left right and centre here.
They are actually saying on ADSL2+ you are not getting any benefit so we have limited you to ADSL.
Now if your sync speed is under 4Meg I would roughly agree, but at 5Meg or better on ADSL, the extra speed of ADSL2+ while not always close to 20Meg can make a difference i..e if you get 5 now, then 7 is at least an improvement.
One key point is that with ADSL2+ they usually allow the upstream to run faster. With the ADSL products there is a faster upstream product option still, but they charge more for it, since it gives your traffic possibly higher priority at peak times, i.e. is intended for businesses.
If upstream speeds are important and line was stable under ADSL2+ then make this point to BT and see if they will regrade you back to the ADSL2+ product i.e. what they confusing call up to 20Meg