You see being a plumbers mate kind of sets you in a single role and you can become stuck as just a plumbers mate.
personally with an apprenticeship you can plan it nvq level 2, level 3 nvq and the gas as part of it, at 17 while you have the time its the best option, by 21 you could be gas safe and highly competent!
My friend went as a plumbers mate, 3 years down the line I'm oftec, passed my acs, will be gas safe in a few months (poor croppie) , have g3, some electrical qualifications and moving fast.
he has his level 2 nvq and is still just pushing pipes on sites.
as an apprentice you will do almost everything and get a huge amount of experience, i know the pays not great but in years to come you will realise that experience would have cost you a 1000 quid a day paying for it.
Don't aim to be a plumber, aim to be a gas and heating engineer! Aim as high as you can
others might have different advice but thats mine 🙂
good luck 🙂