The rules are actually about campaigning in uniform as opposed to doing anything ‘political’ in uniform (you can even wear the uniform of the HAC in the House of Commons!)
Ruth Davidson is ex-service (TA in 32SR) but seems to have had a medical retirement while an Officer Cadet (I am making assumptions here.) However, I suspect she was appointed on the grounds of her civilian “eminence” as opposed to as a “senior serving or retired Military Officer”.
[quote=“Army Reserve Regulations 4.413.e”]Nominees for appointment as honorary colonels can be either prominent civilians with no prior military experience or senior serving or retired military officers. Regardless of whether civilian or military, nominees should:
(1) Have achieved eminence in a field, either connected with the business of the unit or in the region of the unit.
(2) Have empathy with, although not necessarily a background in, military matters in general and, where
appropriate, the reservist concept in particular.
(3) Have links and influence in the local community.
(4) Be able to represent the unit at a senior level in the civilian and military arena, locally and nationally.[/quote]
Because it is 32SR and she was one of them?