As a 10 year Army veteran I say "welcome to the ranks"! Just be yourself, present your case as lucidly as possible and you should be o.k.. Clean pressed clothes, etc. My time in the Army was some of the best years of my life and taught me many things. About myself and about people in general. I still use some of the discipline's I learned in the military, made me a better person to be and to be around. Imho of course. Remember though, you're going to meet some real buttheads there. You have to respect the rank, not the person wearing it. If possible, not sure how it works in the United Kingdom, but get some overseas time in. Travel will broaden your horizon like few if anything else. Make them guarantee it in writing. ANYTHING you are promised, your military occupational specialty, travel etc. get it in writing. Good luck!
(If you don't want to go, listen to "Alices Resturant" and take your cue's from Arlo. ("I want to kill......veins hanging from my teeth.....wanna kill...kill.....KILL!")