I couldn't find it earlier but I think the price has gone up slightly and pretty sure it was this one:
Shop the latest Carerra Vengeance Junior Mountain Bike - 26" Wheel at Halfords UK
www.halfords.com
It's eight speed rather than seven speed like many others are at this price so more choice for upgrading the drivetrain and it has disc brakes, only mechanical but gives an option to upgrade later if needed.
The other one he looked at which is 1x9 with hydraulic brakes was this voodoo:
Shop the latest The Voodoo Nzumbi Mountain Bike is a great bike for kids looking to hit tracks and trails at Halfords UK
www.halfords.com
He asked his son which he wanted and he wanted the vengeance because he preferred the colour