I began flying military helicopters for a living in the first week of 1979 and am still doing so in civilian street. I also hold a CPLA and used to teach close formation in helicopters (no closer than 1 rotor span) and on fixed wing.
In London ATC are now reportedly getting concerned about having to physically separate helicopters from helicopters at the heliport, based on wake turbulence. It's total overkill imho, but as a mere pilot I probably know almost nothing about flying.
"Recommended spacing is..." is still the best way to deal with it. Let the pilot take responsibility and make the decisions.