There are a fair number of threads on both surplus and DoE. Pretty much all of the surplus ones echo what Chris says, which is that much of the army tries not to use a lot of the army stuff, even so they get it for free.
Army boots are heavy, although not a bad price if you find the right place. I'm actually wearing German Army boots at the moment, and they are very practical, comfortable and hardwearing. They also weigh a ton, and even on a Bronze DoE, would exhaust you just walking in them.
Cotswold are doing a pair of Hi-Tec Altitude IV boots for £59 (the mecca of army surplus, Endicotts, are doing 'new' british army boots for £39-49). With DoE discount, etc, you might do even better. Even if you never do DoE ever again, a pair of half decent walking boots are always useful.
Try TKMaxx as well. They often have Karrimor KSB boots (one of the few things that is decent with the Karrimor name on it these days, unlike the Sports Direct stuff), and of coure they will have fleeces, socks, Sigg bottles (my seven year old got one for £2), Craghopper trousers (does anyone buy these full price?), and the occasional waterproof. Lidl/Aldi base layers for running/skiing are excellent (in fact so good I snagged pretty much the last bamboo T-shirts in the shop).
The same goes for a backpack. If you can't borrow one, then look around for something decent, but not too expensive.
Argos has an outlet on Ebay, and they currently have a Vango 60lt rucksack for £21.79 plus £2.99 P & P. As long as its not too big for you, it looks like a bargain (the very similar Contour 60+10 is £50-70 elsewhere). Certainly a lot better than going to Mountain Warehouse (overpriced tat) or surplus stuff (trashed and seldom all that comfortable).
I'd stick with the school tent at the moment. Your not going to get a decent one for less than a ton (I know, I'm looking to buy one myself) unless the fabled Tiger Paws comes back, and you can use that money in better ways.
Spend your money where it will do the most good, keep you comfortable and be useful if you never do DoE ever again. Decent boots, decent waterproof (its rainy in Britain, so buying something that keeps you dry is never a waste) and an OK pack. Alpkit is great - their headtorch is a must buy, and their drybags are very good value.
The only thing which you could get from surplus are mess tins (I've still got ones I bought 30 years ago, and they are dated 1952!), and the german mess kit and KFS are really good.
I found a kit list from an ATC squadron (that brings back memories!) which seemed pretty good, and a quick search around OM will bring up plenty of DoE advice. Good luck!