When I flew the VC10 for BOAC we had two flight planning tables, M.84 and M.86. However there was a difference between indicated Mach and true Mach. For an .84 cruise (indicated) we would file ,825 with ATC. There was a big fuel difference between ,84 and .86 so I would imagine that .87 would have been interesting. Perhaps someone with more recent experience (BEagle?) could confirm the actual difference.
On the other hand I once flew a B747-400 from Heathrow to Tokyo at M.87 (with company approval to beat the curfew) and the extra fuel used was under 2000kg.
Whatever the truth about the speed, the VC10 was certainly a beautiful aeroplane to fly. BTW, I recently visited Duxford and confirmed that there are no plans to scrap the VC10.