Re: "no requirement of 'registration' for deactivated firearms"
"Reversing" a UK-spec deactivation is tantamount to building a new firearm from scratch.
Which is why organised gangs have stopped doing that and are just making their own firearms from scratch (Gun factory uncovered in Sussex) - which is easier and more reliable than doing "bespoke" reactivations of whatever becomes available on the deact/collectors market, and leaves no paper trail back to the white market. Or just smuggling them in from Eastern Europe (60 guns found in car at Dover).
Any metalwork teacher worth their salt could churn out sensible quantities of handguns on a small lathe or mill. This isn't magic knowledge. It's not hard once you've sorted out a pattern.