Both UM and CCX are very high end cars with similarities and differences.
Differences:
UM:
They have their own website...
2" x 4" steel box type frame
More of a sloping rear section compared to the S/C type car
Front end suspension is now a Weaver family designed unit
Weavers are still owners of the company
UM: 
CCX:
4" round tubular frame
Body replicated from CSX3045
Generally used Jaguar front suspension either Series I or series II
Company no longer exists however there is a strong support club of owners
See:
Interview with Peter Bayer
CCX: 
Similarities:
Jaguar rear center section
Ability to use either FE or SB configuration
Both can be found in street or S/C styles
Both cars have tenacious group owners followings.....
BOTH ARE HIGH END CARS!