Keep in mind that JTF recruits from all MOCs. Worry about BMQ before JTF. There's a waiting period before you can join.
Your best bet: find an MOC you could do until you retire if you don't make the cut. Bust your *** and work out. Apply whenever you can. Take any courses offerred. Serve The Queen loyally.
If you want to be an assaulter, I would imagine (I'm by far not an expert in the matter) combat arms would help a lot. They want (again, I assume) people who know how to do heavy ground work.
Just an addendum: I'm not even in BMQ, yet. I do have certain family members who trained PT. JTF2 is serious business. Get fit. And by fit, I mean FIT. Like, run 10k quite easily. Not by any means an expert opinion, but yeah. JTF is more than being good on CoD with the M16 and M203. Just basic heresay. Being special forces is serious business. I know a few people who would beat me dead who wouldn't make the cut.