The size of a pack for a DoE trip is a hardy perennial - part of the reason the rucksacks are relatively large is because they have to fit a relatively bulky (but cheap) sleeping bag in them, and roughly 65L is the average size rucksack that everyone has at home that can be loaned/given to the DoEer. And the sleeping bag everyone has is just about small enough to fit in the sack!
My daughters 12, and I bought her a Vango Nightstar for £35 - its perfectly fine for her (of the six other girls in her tent on a recent school trip, all of them either had exactly the same bag or the 2-3 season version), but has all the compressability of a breezeblock. There is a reason why my sleeping bag is from ME, and cost me a lot more.
If she's a 15 year old girl, she'll probably need a ND (narrow backed) rucksack
https://lowealpine.com/uk/expert-advice/womens-fit-nd/. They are shaped slightly differently, so that the straps wont fall off her shoulders (my wife once borrowed a Karrimor Jaguar, and thats exactly what kept happening). with a shorter torso. If you get a non ND one, it often wont fit, and will be really uncomfortable. Obviously if she goes into a decent shop (Cotswolds have always been good to me), you'll get the chance to try them out with weights in, so that the fit and the back system are right. So she might end up with a standard pack, but I'd start with ND ones first. This a really good guide to sizing, etc
http://www.outdoorgearlab.com/Backpacks-Womens-Reviews/Buying-Advice
Roughly 55-60L would be perfect (think about no more than 20% of bodyweight to lift), and Berghaus/Lowe Alpine would do very well - good back systems, not too pricey, but not as heavy as Vango, and do ND models.
The Berghaus Trailhead is a classic DoE/gap year sack -
http://www.millets.co.uk/equipment/...&istBid=tzil&gclid=CJaWq8X4_dECFe-37QodsJ4Asg - £85 is a good price (its as low as £70 if you look online) , and Millets have a 10% off thing at the moment. Thats 1.6kg Cotswolds are doing it for a £100, but there is the DoE discount, etc. Millets also have the same brands Ridgeway for £100, which is 1.7kg
Lowe Alpine always do good kit, even at the cheaper end, but £120 will get you a very nice 1.6kg bag -
http://www.cotswoldoutdoor.com/lowe-alpine-womens-cholatse-nd60-70-rucksack-e1112669?id_colour=124 - bit big, but still pretty light. If you can get away with 50L, then this is good -
http://www.gooutdoors.co.uk/lowe-alpine-cholatse-nd50-rucksack-p371804 , but there really isn't the backsystem you'd find in the heavier models.
But if your spending £120, then Osprey starts to get into the market, with the Kyte 66
http://www.cotswoldoutdoor.com/osprey-womens-kyte-66-rucksack-e1112633?id_colour=108, at 1.77kg
If your counting up the cost of equiping her for a 2 day trip which she might hate and never venture outdoors again, then you could go for the Vango Sherpa
http://www.gooutdoors.co.uk/sherpa-65-womens-p178257 . At 65L its a bit big but you dont have to fill it, but it is 2.6kg!
If it was my daughter (and it will probably will be in a couple of years!) the Berghaus Trailhead or similar would really suit - at 1.6kg its relatively light, about the right size, has a decent back system and is a decent brand. Its actually the sort of thing I'd recommend for my wife as well if she ever has to go on some sort of work Outward Bound thing (which is her nightmare).
Its a long term purchase, so its something that will be useful in the years ahead. With discounts you might get it down to less than £80. Strangely, its really difficult to find them second hand - you'd think that people would buy them, the kids would use it once and then get shot of them, but it seems that they just get used for other things.
If you can get the rest of her kit list down in terms of weight/bulk, then a smaller/lighter bag becomes a real possibility, and thus an easier load to carry. Sleeping bag and mat are the biggies, so if dad has a nice down bag that will suit, you could lend it, or at least justify buying lighter kit on the grounds that its for your daughter's welfare!
Good luck.
