Hmmm, there doesn't seem to be much information online ( or I'm looking in the wrong place ). The MDHU's are integrated into NHS hospital, but don't seem to have surgical facilities, so assume that would be carried out in the NHS hospital first. lessSSN's have permanent crews, each draft being anywhere between 2 - 4 years (my last one was 4 years), Bombers in the patrol cycle (2 of them) both have 2 permanent crews, same duration periods as SSNs i.e. 2 - 4 years, who cycle patrols, so you end up doing every other patrol.
Submariners can go to the surface fleet but it's difficult to do and not a common occurrence, easy for the logistics branch but more difficult for other branches due to the re-training required.
The MoD doesn't have facilities, as such, for PTSD but every base has a welfare branch in attendance (uniformed branch of the RN but they wear civilian clothing at work) and there are no treatment programmes that I'm aware of. There used to be a psychiatric hospital at Netley marsh in Hampshire but that closed years ago and I think there are psychiatric facilities at MDHU's (Ministry of Defence Hospital Units based in NHS Hospitals such as Derriford in Plymouth) but whether they have wards or facilities I have no idea. Google MDHU and see what pops up.
Discovered there are NHS and private clinics in Scotland who treat ptsd. I will have to contact a few to ask if they treat service personell
Which leads me into another question, which you may not know the answer to or may not be able to tell me ( not sure if it's considered sensitive information). Thought I'd ask anyways. In my story the submarine is going to sink. Once the survivors have been rescued where would they be taken? Rescue ships first for assessment ? Would the injured requiring surgery then be flown to one of NHS hospitals?