The fixed rate of £850 is pretty much it. This is to cover all sorts of stuff - a new suitcase every x years, new sunglasses every y years etc etc etc. You can also add £100pa to that to go towards the travel you have to do that's not for normal commuting eg sims, ground training etc. If your employer pays less than HMRC of 45p/mile, you can claim back non normal commuting that way, but you can't claim both that and the £100 - work out what's best for you.
BALPA subscriptions - you can put 67% of that against tax. I assume that's also true for other unions.
Uniform - don't know what happens if you have to provide your own!
Hope this helps