Some questions answer themselves.
The CAA stated quite clearly that they have no interest in or remit over aircraft that do not fly in the Bruntingthoprpe Victor incident (despite the fact that it did)
How could Just Jane or any of the other myriad ex-mil types for which no one has type ratings taxi (and are often not be airworthy) if a licence was required?
Engineers, as already discussed, have no flying licence. You can bet that a GA engineering company doesn't send its engineers for sim checks to taxi C150s around a club airfield. The law is the same.
Is there a legal requirement for an aircraft to be insured if it is only taxiing? Probably not a CAA one, but again airfield requirements might require it. Doubtless this comes down to insurance.