All depends if you are a Qualified Service Pilot and how many hours you have.
You should be able to get a Class 1 Medical by going to an SMO qualified to do initial issue. There are several in the RAF and it works out considerably cheaper than going to Gatwick.
As for getting an ATPL you'll need to do Groundschool. If you have >2000hrs military time, you can do a Bridging Course (Done by
www.bristol.gs). If you don't, you need to do all 14 ATPL exams, should cost £1900 but you can use £1000 of ELC to pay for it. If you are on resettlement then you can use ELCs for whatever you want. (Bristol again is the place to do them)
As regards getting a ME/IR which is needed to get a Frozen ATPL use one the flying clubs at Exeter. Should cost around £7000, provided you have held a Procedural Green Rating in the last 3 years. Again ELCs can help towards the cost.
Oh, you'll also need a Multi Crew Co-operation Certificate, cost upwards of £2000 before you can legally work as a commercial pilot. Again, you can use ELCs to part fund it.