In London Transport days, a 'general hand' was broadly an unskilled level worker on the 'inside staff' (i.e. working inside the garage, not the 'road staff' who were the drivers and conductors.)
Duties included cleaning buses and things like basic maintenance / odd jobs round the garage, possibly assisting a semi-skilled worker. The job would also include shunting buses within the garage, which (in those days) needed a car driving licence at least. Since the 90's you (generally) need a PCV or LGV driving licence to drive a bus on the public road.
The job also tended towards shift working, and there are some legal limits on what shifts people under 18 can do. Operators (or their insurers) may have their own views on age restrictions for safety critical jobs, although not sure it's quite as rigid as on the railways.
Since privatisation, many companies have changed the job title and roles to suit their own requirements. I believe that Go-Ahead London have outsourced bus cleaning / shunting to a contractor, but not sure who this is. Again, cleaning work tends to be mid evening to early morning, and the hours may be outside what a 17 year old can do.
Go-Ahead London do have a 'commercial' team, who do (or did a few years ago - I'm a bit out of touch but I would imagine they still do) things like rail replacement services, special services for things like Wimbledon tennis and Epsom races, which involve some roadside staff as well as drivers. When I was last in touch with someone who worked there, they did rail replacement for Southern, Thameslink and South Eastern. I'm not sure what the implications are of go-ahead group losing the South Eastern rail franchise.
It's worth bearing in mind that rail replacement jobs can start very early morning or finish very late at weekends and (on the Thameslink) overnight, and having your own transport to get to locations is an advantage although may not be essential. From what I gather, GAL's first line of rail replacement controllers was people who work either in their offices or at garages doing it as overtime / rest day, but think they do have some 'outside' people as well.
First Travel Solutions employ controllers / co-ordinators on rail replacement services for SW Railway and Great Western. (I did work part time for them a few years back when they were First Rail Support) - I can't remember working with anyone under 18 when I did this, but also don't remember there being a rule about this.
Not sure who does rail replacement for London Overground. I think with London Underground, the lead bus operator for each individual job provides controllers, and some may use casual staff as well as full time workers doing rest days.
Generally, you're allowed to be on the books of more than one rail replacement operator as it's casual work, although they would probably take a dim view if you make a habit of saying yes to a duty for one then turning it down because you get a better offer from another, and it's probably not wise to do an early duty with one then a late duty with another although i'm aware it happens now and then - for that matter, it wasn't unknown at FRS for a short early and a short late to be combined - you sometimes get rail closures from say 2200 to 0200 Saturday and then 0400 to 0900 Sunday - I think I did a short late followed by short early (in effect a night shift with a couple of hours off in the middle) like that once or twice.
In terms of approaches to operators, it may be worth being more open ended about what sort of work they might be able to offer, and / or exploring possibility of work experience of some sort in the college holidays. I'm not sure how much part time work there is with London operators - traditionally LT employed full time, unionised staff (what part time work there was tended to be for semi-retired staff) but things have changed.
Also probably the wrong time of year, but I have seen agency work for customer service at Victoria Coach Station and for Nat Ex at Heathrow advertised - may be something else worth keeping a watch for.
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). I'm aware that it may be considered lower skilled than being a driver or controller, but then again I can't yet drive, and doubt any company will put me in a high up position. I'm waiting for a reply from one operator, but if that fails I'll look at other companies.