a few disconnected thoughts -
Different operators have different criteria for taking trainee drivers on - some require you to have held a car licence for 2 years and have a reasonable amount of driving experience. You can legally hold a PCV licence at 18 (although broadly speaking you're restricted to the sort of driving you can do on 'domestic' drivers hours rules - so local bus is fine, long distance coach stuff has to wait) and some operators prefer over 25's for insurance reasons.
But it's not going to do you any harm to have a look at local operator/s' website or give them a call and ask.
I've not seen adverts for bus drivers that specifically ask for GCSE grades - most say something like 'basic numeracy and literacy skills' or something like that. Of course if you're in London, then you're not taking fares and thinking about giving change any more...
Generally speaking, smaller operators prefer people who already have a PCV licence, bigger companies are happy to train people. There are of course exceptions each way, so no harm in contacting local smaller operator/s if there are any near you. A few larger operators have a policy of not recruiting existing PCV licence holders, and take on people with good 'people' skills and put them through their own training process. (You may want to think about what customer care type skills / experience you've got and can sell to an operator - possibly stuff you've done through part time work, or through volunteering?)
You don't say where you are - to some extent, it's better to look fairly close to home - bus drivers' shifts can be fairly long, and there's often split / spreadover shifts where you work both peaks with a longish break mid-day. If you live too far away to go home in the middle, these can be a bit of a pain. And bear in mind you might be needing to travel to or from work in the early hours in the middle of winter.
Most operators who put people through PCV training will expect you to pay something back towards the training if you leave within the first two years. And when I say 'expect' it will be contractual, and some will go as far as treating it as a debt recoverable via county court.
As for the idea of it being a 'dead end job' - that's not entirely the case, but any bus operator has a lot of drivers, some engineering staff and not so many of anything else. There are bus drivers out there who have been doing the job for 30+ years, and many of them are quite content that way.
With quite a few of the plc operators, you tend either to join as a driver, in which case you might make it to first line supervisor or driving instructor sort of level, or you join as a graduate trainee and will get to depot manager or higher positions.
Some operators are better than others at developing staff from driver level. From a combination of observation and anecdotal evidence, Go-Ahead seem to be the best in this respect, followed by Stagecoach, Arriva and then First are about the most committed to graduate trainee path to management (although I may be a bit out of touch with what's happened with them in recent years.)
An alternative that's been touched on is doing bus driving as a sideline - although then you'd have to fund your own training, and may find that it's difficult to get in to without experience and difficult to get that experience...