If he's 12 then you're it'll depend on his height plus inside leg size. He
might fit an XXS/XS 700c frame or a 650c one but at 144cm I'd recommend a 700c wheel bike.
The Wiggins one is a 26" wheel. It's a bit heavy but otherwise OK except for MTB-spec wheels. A lot of Moda's are similar. Probably too small in 6 months or so.
Islabikes are expensive but easy to re-sell and they hold their value. You'd be looking at second-hand though anywhere in your budget.
Does it have to be new? There's little choice down at your price point being honest and, with that in mind, what there is isn't a great starting point. Probably my suggestion would be this Triban 120 in XS size for £380:
https://www.decathlon.co.uk/rc120-road-bike-grey-id_8554264.html
Alternatives if that's too big would be this new Dawes Academy 650c on
eBay for £330:
https://www.ebay.co.uk/itm/Dawes-Academy-Road-650c-Road-Bike-41cm-New-RRP-549-99-40-Off/383419732362
Going second-hand there is this 700c-wheeled carbon SAB (collection only in Derby) for £330:
https://www.ebay.co.uk/itm/SAB-Carbon-Youth-Ladies-Race-Bike-43cm/113864711813
Another in 650c size is this Felt F95 Junior for £225 (collection only in Darlington):
https://www.ebay.co.uk/itm/JUNIOR-F...T-CONDITION-WOULD-SUIT-AGES-7-12/202795588055 My son had one of these for a year at about 11-12 and it was really good.
I'd suggest contact some of the local children's cycling clubs. Lots of the parents there have bikes stored that they would sell if she doesn't mind second-hand.
Look carefully and you'll find a good one. Where are you at?
My son's had these amongst others:
Felt F95 age 11 - cost us £400 new:
View attachment 516720
Second-hand XXS size Argon Krypton (full Campag Record!) age 12 - cost us £600 second-hand!
View attachment 516721
Cervelo S3 (48 cm age 13-14, 54cm age 15-now), both frames bought second-hand:
View attachment 516722
Every bike we've bought and used has been sold on without any problem. They still crop up at races and have been passed on quite a bit.