I won't profess to know anything at all about going to the gym, however what I can tell you is there is more than enough hours in the day to fit in an hour or so in the gym; or for anyone else reading this thread, an hour of whatever hobby you choose, and still have time to do revision / spend time on the simulator.
Two things I'd advise you to do
1) Ask when you get to York if anyone can recommend a gym near to your hotel or the training center (The trainers see lot's of candidates every year, and they will be able to point you in the right direction)
2) Get yourself into a routine from the first week, if you're sim group is Monday morning, Tuesday afternoon, Wednesday morning for example, do the gym session at the opposite end of the day. (Unless you really like getting up early and can fit it in before sim time)