With Shelter, the way I see it, you work, they pay you for that work and you pay Thai income taxes on that income from them. Can’t see any fraud in that. Pretty sure that social media lawyer had other things / the real scams in mind, like cases where people got business visa and work permits from companies they had no connection with whatsoever.
In an ideal world, this scenario (I am in a similar income scenario) should be a prime case for the new LTR visa and it’s Work Remotely from Thailand category. Unfortunately, Thai authorities messed that visa up to a point that it is now only applicable to an almost non-existent group in that very category. If you have steady income from your UK limited, getting employed (with Thai work permit and long term visa) by an umbrella company through which you channel some of your invoices would be a way to go. Check out Shelter for example: