DFC
You seem to have some odd ideas about UK ATC.
It is pointless sending a copy of a FPL to an en-route aerodrome whose Class D CAS you wish to cross. The vast majority wouldn't do anything with the information as you are never sure if the acft would call you or route elsewhere; there is little advantage in having a strip prepared. This certainly was the case at Luton when we had the APP function there. Indeed, once we had automatic data processing we would not have even known such a FPL had been sent as the system only dealt with FPLs where we were the destination or point of departure.
An acft calls for transit (thereby passing an 'abbreviated FPL'), you write it on a strip and you deal with it. Simply. You seem to imply there is a delay whilst the details are entered into a 'system'. There isn't. No is no 'system' into which transit acft details are entered.
Earlier you said if CAS transit was refused an onward clearance time would be issued. If you mean a simply Class D transit then I have never known such a time to be given where I have operated. I might have said 'remain outside controlled airsapce, I'll get back to you in a few minutes' but not issue a specific time. This implies, in the event of radio failure, the acft could then transit the Class D. Wouldn't want that.