Thanks for all this stuff, very useful. I had a chat with an IntCorps captain (in charge of potential officer shenanigans) and he made the same point you did about not knowing where one is going to end up. Since I made this post I’ve rather broadened my horizons so far as the military is concerned, and I’m supposed to be having an interview with someone at the King’s Royal Hussars in the New Year- though goodness knows if that will actually happen. I did strongly consider the RAF and the Navy, but moved away from them as I don’t particularly enjoy flying and would feel rather silly in the RAF, even if permanently on the ground, as a result (more complex reasons than that but that’ll do), and as for the navy I’m not sure I’d enjoy being cooped up in a boat for months at a time. I should also say that the military is only one of several careers options I’m considering, though the one I currently favour them most (having just put my application for an MPhil in). My fitness is steadily improving I’m glad to say, and I’m just about running 2 miles in 13 minutes so making progress (still need to do a lot of work in upper body strength though!)@Cicero63 Most things considered, I'd agree that these days you are better off joining the RAF Int branch - even frankly the Royal Navy, but that's still very small - than the Int Corps. Has the added advantage that you know what you are getting before you sign on the dotted line, which is not the case with Sandhurst.
All things considered, I'd steer away from the military at all. You will be joining a peacetime military, which is inclined to be a toxic environment at the best of times, and these are not those. A combination of factors mean it is very likely to remain a peacetime military for your entire career, going on the stats of how long any individual stays in. It's also committed to a steep nosedive in competence, satisfaction and purpose, and as a junior officer you will have no hand on the stick where those are concerned. It is increasingly a lie that you have more control or influence as a junior officer, than as a soldier. You have better conditions and more responsibility, but those are not the same. Quite likely that you will ignore this, but if you do, at least your five year older self can think "I was warned", which is often the difference between a frustrated five year older self, and a bitter ten year older self.
At an absolute minimum, if intelligence interests you and you aren't the most physical type, apply to SIS and GCHQ first. They also have their problems, but vastly better outlooks and a continuing purpose. Or, as others have suggested, view it as an itch to scratch and apply to the Reserves, where the commitment is less encompassing, and easier to leave. All lawyers live in London ,and 3MI is probably one of the best corners of the Int Corps.