Suggesting it's a CAA requirement is b@ll@cks.
It's mandated in
CAP 642 Airside Safety Management(or rather it's not mandated as CAP 642 is guidance and says "should" rather than "must").
Here are some references
g) All push-back crew members should wear high visibility garments in compliance with current standards;
6.9.8 Irrespective of other measures that are taken to provide a safe environment for personnel working in airside areas, all personnel who will be working outside (i.e. on foot) on the movement area should wear high-visibility clothing.
4 Precautions in Freezing Conditions
Winter weather brings extra hazards which require awareness and more care on the part of personnel working on the aprons, if accidents are to be avoided. Simple precautions that can reduce accident risks should be taken as follows:.....
f) High visibility clothing should be worn in accordance with current instructions.
Mike