Hi Anne
from the evidence it sounds like he was in an anti-aircraft unit in London, before being assigned to a DEMS* unit. He may not have been a corporal MP in the RA but rather assigned security/"provost" duties.
*Defensively Equipped Merchant Ship - for protection against mainly aircraft my merchant ships were equipped with a single anti-aircraft gun, and manned by 3 or 4 RA men. Unfortunately, it is very difficult to track down those who served in DEMS units, but some service records do record which ships they served on.
To apply for his records, knowing he was in the RA should be sufficient
K