As far as I know recording the "start finish times" are not actually mandatory.
However, my Service logbook is based on time in the air. In the civvy world hours recorded are essentially based upon brakes off to brakes on time. See LASORS for the strict definition. Therefore you can as I understand it revise your UAS hours to reflect this taxi time.
Notwithstanding this, it is still the case that whatever "credits" you have on paper you need to demonstrate the level of skill/experience to pass the GST.
I didn't have to claim "hours credit" as such towards my NPPL as I am an exQSP but if you want the latest info on Service/civvy log book comparisons try the CAA themselves or try a post on the Military Aircrew part of Pprune.