don't worry!
I was invalided out with a war pension in the 80's so can offer no relevant admin details. However, I can tell you that no one in the service or the MoD is interested in having you get anything less than you deserve. In my case, I have to say that I think that I was well looked after and received - and still receive - a very fair pension and allowances for disability.
What can be worrying is that it takes quite a while for them to figure out every little bit of what they owe you. If you do retire, make sure that you get every day of annual and terminal leave you can save up, transfer, whatever. In my case, my bosses allowed me some 'gardening leave' which gave me time to enrol in some education to get myself going again.
As it turned out, my entitlements were notified about 10 days after my official discharge date.
After you get promoted to Mister, you will find that the Veteran's Agency are also very helpful. The attitude I have perceived is that they are employed to ensure you get every penny you are owed for your service.
Don't forget, also, that you may be entitled to social security benefits if there are excessive delays in sorting the pension. Most of us feel a bit sheepish about claiming benefits - but DON'T! You bloody well deserve it and have paid the tax so you are entitled to it.
Good luck, thanks for your service and I hope this relieves a little worry.