The world is moving on from ADSL and this needs to involve FTTC as well. People don't just select FTTC for absolute speed but also for reasons of reliability - relatively short cable run between cabinet and premises hopefully means less issues with dodgy copper.
An old comment on ThinkBroadband suggested the social tariff was ADSL only, but a comment today on ISPreview suggests you can have it with FTTC - however the FTTC becomes more expensive.
Also I believe BT are the only ones required to provide a social tariff.