Obviously too late for civpop42, but here is how I transferred fairly successfully to the Civvy Street Lancers.
I met a woman I could spend the rest of my life with (so far!), so, having seen countless cavalry marriages go down the pan, I immediately transferred to the Rape And Pillage Company. (It helped that my fiancee's employer was in fact the Unit Paymaster, with whom I raced yachts offshore for the Army.)
Three years down the line I got myself a posting to the RAPC (now AGC) Computer Centre near Winchester and was trained on IBM Mainframes. Served the four-year time bar (total 14 years) then walked.
A few jobs in IT, mainly Consultancy, and now I am a Staff Software Engineer for a multinational IT megacorporation not a million miles from Winchester (no names, no packdrill, but there are clues).
Army pension for 14 years in 3.5 years' time, mortgage all but paid off. It's been hard, but I managed it.
How can you do it?
Go back to university. Find a link to the IBM Manframe Competition. Get through all three phases. It's hard work and you have nobody to help you, but all the answers are on the internet.
Last year only five students finished the inaugural competition in the USA: they all walked straight into good jobs, some of them with IBM. I have got my own nipper doing this year's (UK) competition and he is currently applying direct to IBM.
The object of the competion (there are prizes to be won but they are by the by) is to get people aware of mainframes: if they can do that, there will be a job for them, because no UK university will teach mainframe. They try to tell you the mainframe is dead. I can tell you they are talking bollix.
Once you are in, you are made.
I wonder if, having completed the Mainframe Challenge, you'd even need to complete the degree? There are two of us in our team, responsible for delivery of a half-billion dollar product and my lone A-level is the single highest qualification we have between us unless you count Civil Service Linguist German (Army) courtesy of the HEC at Muelheim AD Ruhr.
Good luck.