PANS-ATM 8.6.5.1
c) except when transfer of control is to be effected, aircraft shall not be vectored closer than 4.6 km (2.5 NM) or,
where the minimum permissible separation is greater than 9.3 km (5 NM), a distance equivalent to one-half of
the prescribed separation minimum, from the limit of the airspace for which the controller is responsible, unless
local arrangements have been made to ensure that separation will exist with aircraft operating in adjoining areas;
though the UK specifies (MATS Pt 1)...
controllers should aim to keep the aircraft under their control at least two miles within the boundary.
2 s