I used to be an Army Cadet, and the '58 pattern stuff you get given (at least I did when I was in) is all you really need. If you're determined to spend your pocket money on looking ally, then I'd recommend the issue PLCE stuff which can be bought relatively cheaply from any decent Army Surplus shop. Webbing ftw.
Both - webbing for day long warry stuff, chest webbing and big feck off bergan for long long walks in the countryside. But since you probably won't be allowed to do the second just get some pattern 58 webbing off ebay with extra water bottle pouches and a decent tan/brown day bag and you'll have everything you need.
Here is a site that sells some of the slightly strange and off the wall stuff that would be cool to have but you obviously would never be allowed to use.