The annoying thing is you really only need a pump clinic, where the iMEDicare rep demos the pump on you and selects the size you need, and that probably doesn't have anything like a 7 month wait. You leave with a sheet of paper which tells your GP which parts to prescribe. You could try explaining you are only after a pump clinic at this stage and see if that can be done sooner.
The NHS prescription part numbers your GP needs to prescribe are 15019 (pump and accessories) and 15222 (3 spare Surefit constriction rings). The 15222 would ideally be on a repeat prescription on demand. You could try asking your GP to simply prescribe these, if he/she will. The constriction rings are not used for penile rehab, only for having sex.
15019 is the large size. The pump comes in two sizes, large or extra large. I don't know the extra large part number off-hand, or what dimension erect penis requires it.
iMEDicare will provide free training for anyone who gets the pump via them, privately or NHS prescription. If you don't get a pump clinic session in person, they can do it over a video call if you have the pump at that stage. Their training is very good, much better than I've heard of some ED nurses.