I picked up a 26″ wheeled Spesh Hotrock, with a 12″ frame. Probably paid over the odds for it but it (£100), was the frame I was after though. Slowly collecting bits together. Got some Exotic rigid forks to go on. 1×9 drivetrain, some Smallblock8’s. Wheels next, just need to pick up some disc hubs. Already have an old set of Jucy’s to Re-build. I built up a 15″ version for the Wife. They come in really light, Wifes is about 22lb. Probably looking at about £250 including the frame. Best thing is the light weight, that’s one of the things kids love about Islabikes.
Halfords Carrerra range are worth a look too. Having taken a set if luggage scales (seriously) into our local Halfords, there is less than a pound & half between a £160 20″ wheeled Carrerra Blast & my lads Beinn 20, at £280. Lose the daft suss fork & they’re about the same. Your paying for kid specific parts though on the Isla, & if you can afford one, buy an Isla as you’ll lose less when you sell.