hey man, the place in which i worked at until recently had someone who was a reserve for 3MI, they might be worth checking out if you are in London. It helps greatly with the recruitment process and timeline if you already have some form of clearance, preferably DV but SC is good also, especially from a government agency/civil service department.
We work with some of these lads and the royal signals on the regular, and let me tell you, they are extremely switched on and on the ball, they know exactly what is happening in all theatres of the world. we also, from time to time, have a JNCO on site, as they are expected to do a stint with my department, and their work can be read by the prime minister.
If you want to go the reserve route, then that is also a very good choice, as the unit is heavily operationally focused and many of the reserves have done tours and placements. it is not the easy route by any means and you will have plenty of opportunity to put yourself forward for service as a reserve. There are other detachments if you are located elsewhere, but the unit is highly selective, so be prepared to hear that you are not necessarily what they are looking for. I have also heard that its better to join as an intelligence soldier, as opposed to an officer, if you want to do the nitty gritty intelligence work, rather than managing a team of soldiers.
It looks like a really solid unit, and I have looked at joining them myself. However, i have a 4 PARA Company in the town next to mine, and the other units are further away, so i may have a crack at that.
good luck with whatever you decide to do!