<<How many of those 20% under spaced vortex wake encounters happened in VMC when the IFR pilot was continuing a visual approach under the control of the Aerodrome controller when the spacing is only a recommendation? And how many occurred on the intermediate approach?>>
ZKDLI.. The answer is just about none. At Heathrow radar control is employed about 99.9999% of the time. Visual approaches are quite rare and if vortex separation is required the controllers will usually maintain control to ensure the correct spacing. Never in all my time at Heathrow did I ever "recommend" spacing to a pilot and I don't recall it ever happening. At smaller airports with fewer movements it may be common, but not at Heathrow.
You further wrote: <<Some of the spacing at Heathrow is done under the reduced separation criteria.>>
When does that occur?? Are you referring to the time from which speed control ceases. ie 4DME? Otherwise I know of no other time.