If you are after surveillance then go to Peter Jenkins in Harrogate. ISS Ltd I believe. His reputation is good and its a no nonsense real grounding from someone that has specialised in that field for many years. Recommended by the Institute of British Investigators.
I have just done my CP course for the SIA licence in July and I am waiting for the certificates from City & Guilds. While I am waiting for the SIA to process the licence I will probably do that course myself as I really want to work in surveillance.
There are some other threads that cover this subject quite well and being military, Task may well be good for you. Are you intending working high risk or would you rather work corporate. I take it from the desire to specialise in surveillance that you are not looking for high risk zones?