PS please try and talk her out of any of these organised BUNAC / STA / similar tours, especially when they want you to pay for volunteering. They really are money for old rope - you could organise the same thing (or a better version thereof) yourself for far less money. The idea that they somehow offer extra safety and security is something of a misnomer, as the things that go wrong are either a) so trivial she can deal with them herself b) require the help of the British Embassy (e.g. losing passport) c) involve some kind of traffic accident, robbery or other crime that the company cannot protect you from d) require a travel insurance claim
In none of the above cases are STA / BUNAC / similar going to be of any help whatsoever. They cannot prevent things happening in the first place, and there's very little that they can / will do to rectify problems after they do happen. At most they are there to point out travel opportunities for after any organised trip and sell you additional overpriced tours. A better way to get that info is through reading the Rough Guide to Timbuktu (or wherever), talking to other backpackers and owners of independent hostels and (dare I say it) the internet.
PPS working full time in a god-awful shift job is character building (50 hours a week behind the bar in a rather rough pub chain did me the world of good after leaving a somewhat middle class school) and so is living in 32-person dorms, attempting to cook in hostels where there are 6 gas rings between 410 residents, negotiating foreign bureaucracy, getting bedbugs, cockroaches and learning to haggle with street vendors. I think I've just managed to make the travelling lifestyle sound spectacularly grim, but in reality I rather miss it!
PPPS I don't want to give the impression that it's common for stuff to go wrong whilst travelling. In my experience, and that of friends, it's very rare for anything to go spectacularly wrong. I'm struggling to think of things that actually went wrong during my trip, and the list comes to having my phone stolen (well, I might just have lost it, but it was cheap so I didn't care), slipping and bruising some ribs, and (separately) spraining my ankle. That's in the space of a six month trip, travelling solo, including a week in Malaysia.
Indeed, she should try and change her entry year now. It's a lot easier to change from non-deferred to deferred entry than the other way round, especially at a late stage.
As of 2010, as soon as you finished your last exam all child benefit, tax credit and whatnot finished, but you could claim JSA in your own right, albeit at the lower rate given to under 25's. This was the case whether or not you were planning on going to uni in September.
Telling her that she has to get a job (with a suitable number of hours!) before results day if she wants to take a gap year is a good idea. Too many gappers leave it too late to start looking for a job, and then moan that they haven't got enough money to go travelling. In some ways, the fact that she has just left school will be an advantage to her, but whatever she does she should not mention that she plans to go to uni. Employers are looking for people who they think will stick around for the long haul, rather than just being a stopgap and a student (or skilled worker who's been made redundant) just doesn't fit the bill for that, I'm afraid. She also needs to not be remotely fussy - bar work is about the height of most gapper's ambitions. Agency work can be a good way to get her foot in the door in terms of experience. I got a job three weeks after I left school (2010) with bugger all experience. But by that point, I had applied for ~150 jobs. I then funded 6 months of independent travel in Australia without parental help (and I worked all the hours god sent to pay for it!)
She is better off trying to change her entry year beforehand, simply by ringing up the department. I did this, as far as I remember it was a matter of speaking to the admin worker at the other end of the telephone (might have been more to it, can't remember, 10 years ago now). Going through clearing is not a brilliant idea as far as I am aware since she is looking at RG popular courses & places on those won't come up in clearing.
I think you are right that you won't get tax credits for her once she is an unemployed adult rather than a dependent in education. Definitely won't get child benefit.