By the looks of it, you are getting quite a lot of phenolic plywood for your money, which isn't exactly cheap, and is more stable than most other wood boards.

Yes, £400 could buy you all the metal you need to potentially create a better machine, but then you've got to learn how to work with metal and probably spend more on tools to be able to work with it or pay for somebody else to do it, whereas that £400 gets you a basic machine that needs a lot less input to get a working machine.

You already have experience of CNC, you have obviously done your research, and you have a reasonable expectations of what the machine should be capable of, so I personally don't think buying that kit would be a bad choice for you. However, I would be looking to try and find somebody who has already bought one, to make sure the kit they received lived up to expectations, and that they never got supplied with a bunch of poor fitting parts.