VIKING=XL=EXCEL=SABRE=CALEDONIAN=PEACH=AMBASSADOR
Not quite...
Caledonian & Peach Air became JMC which in turn became Thomas Cook. Nothing to do with XL.
Can't speak for the old Ambassador but I think you'll find it has nothing to do with XL either, long before those days in fact.
There is a certain connection with Viking & XL/Excel Airways however.