For your info for the B767:
FAA VMO 360 MMO 0.86
CAA VMO 360 MMO 0.84
The FAA requires that a margin of no less than .05 mach exist between Mc and Md. Md for the 767 is 0.91 so 0.91-0.05 = Mmo 0.86
The UK CAA requires that the margin between Mc and Md be sufficient for an encounter with a head-on gust. For the 767, a head-on gust would have a velocity of 45 feet per second EAS at Vmo/Mmo. This equates to a 0.07 Mach margin between Mc and Md therefore 0.91-0.07 = Mmo 0.84. This gust value is determined at an altitude of 25,000', the lowest altitude at which Mmo could be achieved. At higher cruising altitudes the max gust value would be lower.
In addition to the above the FAA and CAA have differing ways in specifying buffet limited altitudes. As the FAA have no specific requirement for manoeuvre margin to buffet Boeing have adopted 0.3g as the min manoeuvre margin. The calculation assumes a cg value of 25% MAC a typical airline in service CG. If the actual CG is fwd of 25% the margin will be slightly lower than 0.3g. The CAA require 0.3g but based on the more conservative CG of 7% MAC which thus results at average in service CGs, in a reduction of up to 1,000' in cruise altitude capability.
Here at RBA we operate both the B757 & B767, the former are under FAA regulations and the latter under CAA. There are a number of other small differences between the FAA and CAA but I hope you find the above of interest.