Without knowing their real situations...
I believe the guy building an engine may have qualified for a CAA "D" licence - but the CAA don't issue those now!
The guy you say is doing maintenance may actually be doing "B" licence work. Similar story.
In either case, there are things called "Exemptions". I worked under CAA approvals and exemptions when I worked for a UK-based flying club in Germany!
Ask them if they can do Annual Star Maintenance, or recommend for a C of A.