RE: Need advice for starting a career in Aircraft Maintenance!!!
As someone who went through the apprenticeship route in Aircraft Engineering, I would definitely recommend this route. Although you have to remember there is no guarantee of a job at the end of any apprenticeship scheme the vast majority of apprentices end up being employed. It offers the best mix of theory and hands on practical experience.
As regards you age, do not worry too much about that. Yes, years ago a lot of apprenticeships were only open to 16-18 year olds but that has long gone and now there is quite a wide spread of ages [plus I think it is now also illegal for the employer to state an age bracket].
I would think very carefully about enrolling on any further courses that could cost you a lot of money and not necessarily get you any further in you application to get in at Mechanic level.
With regard to BA they are recruiting again this year, so it might be worth trying again, as I know from personal experience that the current age profile of the existing Engineers within BA is very high [ie they will be losing many engineers in the next few years due to retirements]
Also note at BA they are offering two apprenticeship schemes one is the traditional Aircraft Mechanic the other is an apprenticeship geared towards the cabin, while the second may not sound that exciting, it gives you another opportunity to get into a large Company like BA and once in, there are always options to apply for jobs/transfers internally.