I'm pretty sure you could wear a kilt, i'm more concerned that you are required to wear a suit. Were you instructed to bring one with you when you joined? An half decent off the peg suit is going to cost you about a quarter of your take home which i think is a little unfair.
In regard to your kilt question, ask you Chain Of Command.
When I was at Harrogate we had some irate parents ring up complaining "my little XXXX has been told to buy a suit-why?". We told them;
1. He has been told to GET a suit (old uncle albert's demob suit was seen a few times).
2. Asda will do a suit for Â£30 and oxfam shops for a fiver
3. He needs it for church parade or court-and in your son's case we are unsure which will occur first!
Usually did the trick and all managed to be in suits by the required day-and no there were no kilts to be seen
A kilt would be acceptable as long as your surname, accent and home postcode bear a link to Scotland and, you're big enougn to piss off all the small minded English t*ats who would try and extract the urine! you stand up for your minority rights!