That really depends on what kinds of things you want to make. After all, you want the laptop you're running to be able to play what you're making once all's said and done, otherwise testing it might be a bit difficult.
With a 3D game, you're going to want something with a discrete graphics card (not integrated) and 32GB of RAM, which will set you back more than £500. Indie games with pixel graphics can focus mainly on the RAM side of things.
For the latter, this doesn't look too bad, but it's £600 (Club386 affiliate link, unsure if that's against the rules. Feel free to delete if so).
https://amzn.to/46FpEPX
I wouldn't recommend anything less than 32GB of memory if you can help it, but there are some good deals on 16GB if you want to risk it:
https://amzn.to/3sVjsFE
As for storefronts, I use Amazon the most because they do have the best deals. Box.co.uk, CCL Computers, and Laptops Direct regularly have price drops on laptops. Scan and OverclockersUK less so, but they're very reliable with fantastic customer service.