I agree with the suggestions that second hand is the way to go. I'm lucky enough to never have had second hand bikes since I was a teenager but in reality any bike is second hand once it has been ridden and they all need maintanence and repairs almost from the start.
My oldest son has been the lucky one who gets the new bikes and this means son #2 has spent all his cycling career on hand me downs but this has never bothered him as they are good bikes and to be honest, any repairs we do are all part of the fun of bike ownership and they both help out and learn when it is repair time. As an example, in recent months both boys bikes have had new sealed bottom brackets fitted, new chains and the youngest son's bike needed a freewheel as the original chain had been left far too long and there wasn't a hope in hell the new chain was going to run ok on such a worn drivetrain.
This just goes to show, even bikes bought new by yourself still need repairs. The knack to buying secondhand is being able to spot the lemons where everything is about to become due for repair/replacement at the same time as this can quickly negate the savings made from buying secondhand. Having said that, there are plenty of good condition used bikes out there and if you pick the right one you will get a lot more bike for a lot less money.
By the way, full suss Apollo with disc brakes for £149
. At that age, rigid or just front suspension and cheap reliable V-brakes are the way to go (finding a junior MTB without suspension forks is not easy!).