The CAA are not proposing that the separation can be reduced, they are saying the methods of ensuring the 4 miles minimum can be unit specific.
No, they aren't saying that at all. The document is clear that airport operators can propose a different separation standard (with suitable procedures) as an alternative means of compliance (Section 4):
"The CAA will consider an application from aerodrome licence holders to apply
an alternative specific Heavy/Heavy aircraft departure wake turbulence separation."
(and would require)
"In addition to promulgation of
the alternative separation standard in UK AIP AD2, evidence that aircraft operators were aware of and content with the local variation."
That's "alternative" as in
an alternative to 4nm separation (subject to a safety case, obviously).