Train driving does not require any high academic achievements, but you need to be self-disciplined, feel comfortable working alone for long periods, assume responsibility and be able to maintain safety-critical procedures without being constantly supervised. If you happen to be a clumsy creature, the job will kill you, so forget it! Beyond that any sensible and reliable person can become a train driver. You have to be absolutely aware of the great responsibility you take in order not to cause an accident so as a train crash driver. What you derive from the job depends on how deep you get involved in the activity and how important it is to you. It can be very rewarding and is far better than many other occupations; however, if you prefer to stay bored and disaffected you'll find plenty of opportunity for that too.
The obvious places to search for a train driver position are the websites of the Train Operating Companies, many of which have an 'Employment' page. Remember that few train driver positions are widely advertised (and if any, the adverts are likely to appear only in the local press relating to the depots concerned) and the few to appear may attract a large number of responses, so it's better to approach the companies personnel departments for request. Most train driver jobs advertised in the railway press (most likely in the 'Rail' magazine) are for qualified drivers. It should be borne in mind that there is a semi-official agreement between TOCs not to 'poach' qualified drivers. This agreement is not always observed, especially by freight firms, but even so there is nothing to prevent drivers from applying for a job in another company. Due to this fact some of the advertisements supposedly for 'Trainee' Drivers may be come-ons for qualified drivers to apply.
In reality, you will probably be expected to start as a Conductor, so this is probably the job you should be applying for to begin with. If no Conductors vacancies are immediately available you might consider joining up the refreshment trolleys with some of the regional companies. It might seem a long way from driving trains but it gets you out and about on the trains and gives you a chance to show willing. It may even be possible to do this during the summer holidays while at college. Prove yourself reliable and you could well be on the next Conductors - even Drivers - course.
Freightliner, GBRf and DRS recruits qualified drivers from within the industry, so there isn't much point in applying to them except perhaps for a Train Person's post. Virgin, Anglia, EWS and a few others sometimes take on train driver 'off the street'. Probably your best bet for fast-tracking from conductor to driver is with one of the Regional passenger operators at one of their big city depots like Birmingham, Manchester or Leeds.
If you pass the tests, the interviews, the medical and you get a train driver job (and remember that whatever you hear on the grapevine you haven't actually got the job until you've got the letter in your hand), there are several months of training and examinations ahead, plus route learning, before you get to drive a train on your own.