I'm not sure how they'd view it to be honest - it sounds like you'll be doing something useful, and if you just really play on the skills you'll gain then that can only be a good thing. As long as you've already got your medicine related stuff, you should be fine. Thats just a guess though, the only way you'd know is by emailing and asking, particularly about the places for deferred entry.
I'm just thinking logically, but you'd have thought it would be easier to add deferred entry than remove it. I know my university has a policy that is something along the lines of 'if you decide to defer, tell us straight away and we'll see what we can do', and I'd imagine others are the same. You are much safer sticking to your decision than trying to change things, just in case they can't do it. But again, I'd email and ask to be sure.
And as for if you don't get an offer, its fairly likely, as you probably know (the odds are against you I'm afraid) so if I were you I'd rethink your plans to allow for doing the UKCAT, BMAT and any interviews. Perhaps if you started your year in late August to give time for the UKCAT before you go, and just plan for the fact you may just have to hop on a plane back for an interview. Either that or in your reference give your availability for interview, and plan to be home for the Christmas, Jan ,Feb, March time.