At LHR by and large it wouldn't work due to their being a service road in front of T5A that's in constant use and is unable to be crossed

At GLA you go down a set of stairs from the gate and out to the back of the plane.

At EDI I think it's gate 7 (The one closest to the lounge by Pret) where you can do front and back I believe.

At LHR the times I have experienced .the rear boarding/unloading takes place is when busses are in play and the inbound has used a non domestic compliant gate. When I say times this has been 50% of my T5 domestics in the last year.