if you don't have a vat receipt your stuffed if you ever take it out of the uk ever again. You can't even return with it without running a risk of getting done.
I,m not really worried about entering the Uk, as I would probably just fly back from France to my home field non stop, so no problem with customs etc, and even if the Uk authorities did question the issue I think I could argue my point with the plane safe and sound in the hangar, what would concern me is getting it impounded in a foreign country as they would have me over a barrel for any fees they felt they wanted to charge.