My understanding is that Gloucester only do Aerodrome and Approach Surveillance ratings. So Gloucester provide NATS with controllers who will end up working at airports and at Terminal Control at Swanwick, specifically the Approach positions for the various airports in the SE that do not provide their own Approach function from positions in thr tower building (LHR, LGW, LCY, STN, LTN).
Most of NATS' own training at Whiteley (near Swanwick) is for Area, so controllers with that rating will end up working at Prestwick or Swanwick (Area Control) and the non-Approach sectors at Terminal Control. I know there was a period where NATS only trained Area, but that may have changed by now.
Moving from one discipline to another is very rare now. I wouldn't choose your preference based on an assumed ability to move later in your career.