I always wanted to fly RAF but Medics said no. Would have gone ATPL but simply couldn't afford it. I simply accepted it was not possible, but sometimes wonder what might have been ...
Then again, like you, I also wanted a commission in the RAF. I went for FC and have not regretted it (mostly
).
I have to say, in all honesty, that if you want to fly then you must take whatever opportunity (mil or civ) that presents itself. RAF ATC may provide you with time and money to make progress towards ATPL, and I would guess you will be a better pilot for it.
The other way will get you to ATPL quicker, but at what risk? I suspect that with the way licensing is going, the ATC ticket may provide a very useful fallback, either to return to the RAF or take to Civvy Street .
Whatever decision, good luck!
STH