With him in the RAF then not much chance of splitting up as you will never see each other enough to get peed off!
If you are buying together next year then less of a worry splitting up wise so I will revise my suggestions :)
If you are living together as partners then how about a joint account? Both contribute into it, pay the mortgage and bills through it etc. This would not be an issue from a tax point of view as the money would be going into a joint account.
Um, think you have to be careful, not from a tax point of view but from a splitting up and your lovely boyfriend trying to take half your flat!
We do not like to think of worse case scenarios and I hope you and your lad live happily ever after but relationships do go sour and I am not sure at what point a live in lover becomes a common law partner so maybe someone else here can help out and let us know.
You might be better going down the route of him being a lodger as I am sure you can rent out one room without tax liabilities. Get him to sign a contract (for tax purposes, don't need to tell him it is for your own protection) and this way if things do go worng he has only ever been a lodger and has not been "contributing to the mortgage" as that may give him some hold over your dwelling.
Sorry to not answer your question but thought you should take this into consideration.
