The accelerated apprenticeship, with a starting wage of £31k is probably one of the best job offers I've ever come across in the public or private sector. It's such a good offer that the only worry is how long it'll be before some penny-pinching government mandarin decides to axe it.
If you think you may struggle with the academic entry standards for AA, or are fed up with being in school, then standard Engineering Technician is still a good way to go with sought-after transferrable skills and qualifications. Promotion can be fast if you are good at the job.
What age to join? That's your shout but you can be getting £880 a month in your pocket from age 16, twice that by age 18 or you could join up after uni, having racked-up an average of £50k debt compared to earning over £100k had you joined aged 16 or 18. Engineering Officers join on £6k less than accelerated apprentices, but currently get a £27k joining bounty (whether that is still offered in five years is open to conjecture).
My tip, aim high, go for the accelerated apprentice scheme, you could still get an engineering degree (free from tuition fees) and later become an engineering officer, if that's what you later aspire to.