Genstabler, dedicated service constituencies have been suggested before, as you probably know. One difficulty is that either service voters would have two votes each (nice, but dream on), or they would under your suggestion no longer be allowed to register either in the local constituency where they currently live, or in the constituency where they would be living if they weren't in the forces.
Service constituencies aren't on offer, but any case I wouldn't see a service MP as any substitute for a service complaints commissioner or ombudsman.