If the unmodified FMC speeds were used by the 757s and 767s operated by one British airline, then the 757s would come down at 0.79ish/277kts and the 767s would use 260kts almost the whole way down.
In practice most of us modify this to something of the order of 0.8/290 or 0.8/300. In turbulence we have to use 0.78/290 (Flying Manual restriction).
As I am sure you know, speed control is best used to maintain a gap rather than create one and it only works really well if the speeds are set up before the descent commences. I appreciate that as a South Banker you often get us fairly late from the French, so that will not always be possible.
Perhaps 757/767
Fast 0.8/320
Med 0.8/300
Slow 0.78/270
(As an aside, most of dislike flying much below 0.78. I think this is partly because we rarely operate there and partly because we need to check both the Mach graph and Vref+100 at those sort of speeds).
Hope this helps.
G W-H