I think this must be a rather rare later version of the Miles PE, as the earlier ones did not have the plumbing routed to the rear side panels, which of course meant that the main pump could be more easily engaged with the escape ladder still in place.
This configuration - which has the advantage that the pump can be much more easily operated with the wheeled escape ladder in place - seems to have first appeared in the mid nineteen sixties. It was in evidence at the 1966 commercial motor show on both the four Magirus escapes fir Glasgow and the first ERF dual purpose which went to Newcastle and Gateshead brigade.
Older approaches were to mount the pump amidships, but this restricted the amount of space for the first aid water tank, or to simply rely on slipping the escape if it became necessary to use a rear mounted pump.
Glad to hear it, it did make a lot of sense in the time of the wheeled escapes. The other idea - a front mounted pump - seems to have been confined to the less than a dozen Leyland Firemasters introduced in 1960. Of these, two were TLs, two PESTs and only six pump escapes, however. Based on a shortened bus chassis with an under floor centrally located engine, it seems that it was just too odd a design to ever be very well received. Engine access requirements also precluded fitting a large water tank, too - which did nothing to add to its appeal.
Added by Rob Johnson on 04 July 2023.