ADSL is at its best, 24Mb down, 2.5Mb up (using annex M).
Technically in POTS based ISPs, next is VDSL (Very high bitrate digital subscriber line) which is 200 Mb down (not sure on upload).
However, not many companies have released it. I think that soon, we are going to have a wave of ISPs releasing bonded ADSL2+ services (not dual ADSL, actual bonded - so your equipment thinks it is connected at 48/5 instead of 2x 24/2.5) as most already have this service available to them on their DSLAMS and it is only a case of the user buying a ~£100 modem/router... (anyone who wants this sort of service, would be happy to pay that!) This is also cheaper compared to the many thousands it would cost to roll out VDSL equipment.
Also, the best lines available to date are fiber, direct to your premisis and terminating at a data center - it is available to anyone with the cash and up to many GBs in speed, just don't expect it to be cheap.
Alternatives to look for are cable based services. Most are rolling out DOCSIS 3 based networks at the moment which can rival VDSL at over 160KB/s.
(VDSL2 is also better than VDSL, but if your provider hasn't rolled out VDSL, don't expect 250Mb Up/Down to be coming to you anytime soon!)