1) How old are you?
2) You don't say what you do in the RAF. Are you a pilot already?
3) What flying experience do you have?
Sorry, not an expert like scroggs or PP but here are my thoughts.
If you join the police you will have to concentrate on your basic training and if you are not giving it 100% you may not make it through your probationary period. Secondly, are the police going to be happy to let you have 2 lots of 3 weeks off for ground school brush-up and then 3 months off to complete your flying (3 months is assuming you have a PPL or RAF equivalent, if not you need much more training.) All the while that this is going on you will be getting older and if you're the wrong side of 30 that could be a problem. Remember that once your training is complete it can take a couple of years to land that first job so you have to ask the question whether the police will accept you back after 3 months when they must realise you'll be leaving any minute? I know they have allowed experienced police officers to do this but a newbie, I doubt it.
To my mind (and I was a modular guy) if you have no flying experience go fully integrated to Oxford or Jerez because from what I have seen that is your quickest way into this industry. If you do well with these schools they will recommend you to BA and assuming you are successful you will be on more than twice as much as the police were paying so over a 3 year period you will be better off. It is of course a gamble, but Oxford do seem to work very hard to get their best students into airlines so it all comes down to the question of are you cut out for this game.
If you do have flying experience then I would look carefully at where I did my CPL/IR and speak to schools with a history of getting their students into airlines. I would still be tempted to go full time with Oxford if you could possibly afford it (that wasn't an option for me) because the quicker you are trained the quicker you can get a job flying and if it's flying that you want why waste time going down the police force route.
It all comes down to what you can afford, I was mortgaged to the hilt had no interest in selling my house and could not get another loan so I had to go modular (and 3 years after starting my ATPL studies I still don't have a flying job).
If the worst happens and you lose your medical can you not continue whatever trade you had in the RAF?
Just my thoughts, ready to be shot down by the anti-integrated brigade.
SW