Well, its a start, just not a very good start. Assuming you don't have to do a whole 6 months and its a daily rate:
Approximate "Tax Bill Refund" = £12 per day.
My approximate tax bill = £40 per day.
While better than nothing, I'm not exactly leaping for joy.
Wouldn't it be much simpler to put it on the tax return:
Q: "Are you in the Royal Navy, Royal Marines, Army, Royal Air Force?" Y/N
Q: If Y, how many days did you spend in an officially designated operational area (or words to that effect)?