From UK MATS PART 1, Section 1, Chapter 3
Standard vertical or horizontal separation shall be provided, unless otherwise
specified, between:
a) All flights in Class A and B airspace;
b) IFR flights in Class C, D and E airspace;
c) IFR flights and VFR flights in Class C airspace;
d) IFR flights and special VFR flights;
e) Special VFR flights;
f) Participating IFR flights in Class F airspace;
g) IFR flights in Class G airspace being provided with a service by an approach control unit;
h) Aircraft participating in the radar advisory service.