Don’t know about prices as I don’t live in the UK anymore but…..
My 8 yr old is riding a Spesh Hotrock 20″ with tyres upgraded. He as no issues riding it other than the smaller wheels on log rolls. He can keep up with pretty much anyone in his age range and a good number of adults. He stacked it recently on one of our local trails and a couple of guys roiled past moaning about kids on “their” trails. I suggested he try to catch em up after he’d picked himself up, 200 metres later he was buzzing their back wheel! Weights obviously not too bad and he gets some movement from the crappy suspension fork.
My older girl (9)is on a 24″ Cannondale which is about to go on a diet. It’s 12kg but I reckon I can lose a heap of weight. Got some Stans Crests and my old XTR group for it, aiming for well under 9kg.
Oldest daughter is 11 and rides a 26″ Trek which is 15kg!!!! Again it’s on a diet, much easier here as I’m sure we’ve all got loads of old 26″ stuff in the spares shed. Stans wheels again and a 1x XT group paired with a Reba. I reckon I can get sub 10kg easy enough?
It’s definitely harder with the smaller bikes to find ways to lose weight!