I can confirm what you've got here PN. Last year I claimed back 6 years, albeit since I'm long out of uniform, for civvy journeys. But mil or civ, it's the same tax rules.
Claimable rates depend on cc of course. So a 2.0l car is allowed to claim 40p per mile for 10,000 miles and then 25p after that. You calculate the difference between what you were paid by the Services and that which is permitted under tax rules and claim the difference. You'll get back the proportion of the difference that is your max (marginal) tax rate - 22 or 40%.
A point worth noting is that if you set off from home and didn't go near the station (or base!) in any meaningful way, you can claim the HTD mileage too, as it doesn't in that case count as HTD. Don't take the p on this one though. If you drive past the gate, you can't claim; if you go a route not within a few miles, or which is in a different direction altogether, that's fine. I checked this with the IR helpline (who are in fact surprisingly helpful - to the point they phoned me back to correct something they got wrong!).
Correct too about being driven by someone else to/from the likes of a rail head.
I keep track of miles done by category for every journey, by date and from/to, so if challenged by the IR I have the evidence to the granularity they want.
I got several £100s back. Go for it
