As others have said Jamie, read the Engineers Licence Guide Document on the CAA website. This tells you the basic requirements and the routes to gain an "A", "B1" or "B2" AMEL. For the "B" tickets, that allow you to issue Certify Release to Service certificates in the tech log of an aircraft or base maintenance CRS (WITH the correct aircraft type ratings and company approval from the quality department of the PART 145 organisation you work for), it is going to take you 5 years of experience proof, including passing all the theroretical exams (17 for a B1, a couple less for B2) before the CAA will think about issuing a ticket, if you take the exams via self study. If you did a full time 147 course like the ones advertised by AST, LRTT, KLM Tech College, etc (two years in length) the minimum requirement of experience is two years working in a PART 145 organisation. The CAA do exempt experience time off for persons who have a suitable and proven record of military aircraft experience, but they still must do a year minimum, working within a PART 145 enviroment. The "A" licence is less time, but is limited on what task can be certified, and is only for self certification of these tasks(the ELGD specifies this). Some MROs seem to be starting schemes where experienced tradesmen from other sectors (such as motor mechanics) have been introduce at positions below mechanic and will receive OJT as well as oppertunities to study in a 147 based enviroment (night school). They have no time exemption.
As an afterthough, the military will soon have leased aircraft that require 145 maintenance. While 5 Sqn are sorted, Air Tanker will invariably require military licenced guys as well. If they are all seniors remains to be seen.