A new standard for ADSL called ADSL2+ provides extended bandwidth. This follows ITU G.992.5 and provides up to 24Mbit/s downstream and 1Mbit/s upstream.
A variation of the ADSL2+ specification called Annex M allows up to 24Mbit/s downstream and up to 3.5Mbit/s upstream. In the UK we cannot achieve the full 3.5Mbit/s upstream as there is a frequency plan that must be followed on all phone lines to avoid interference. Therefore, in the UK, Annex M allows around 2Mbit/s uplink.
It is important to realise that the above is a technical statement about the ADSL2+ technology. An actual service will achieve a sync speed (which includes various overheads) depending on the line length and quality and other factors, and may even change over time. To assess the likely speed of your service, please use the availability checker.