I've just spent a while looking carefully at it, and there's absolutely nothing in the Road Traffic Offenders Act 1988 that allows endorsement of a license for the offense of "Cycling when unfit through drink or drugs." - the only punishment listed is a fine of "Level 3 on the standard scale".

I'm not aware of any more recent legislation overriding this.
The Act is online at:
<http://www.hmso.gov.uk/acts/acts1988/ukpga_19880053_en_1.htm>