>> recorded their state there as “non-dom” or other living-abroad type, and then never registered in Spain; hence avoiding taxes both in UK and Spain.

I mean, while I enjoy this idea as much as anyone, I don't think it's possible. If you tell HMRC that you don't have a British tax residency, then they will insist(very persistently) that you tell them which tax residency you do actually have, and with EU countries they absolutely do exchange information on taxpayers. It might take a few months or even years but they absolutely will catch up with you eventually.