MATS Part 1, the controllers bible says
The conspicuity code, 7000, together with Mode C should be displayed by all suitably equipped aircraft unless:
a) they have been assigned a discrete code;
b) they are flying in an aerodrome traffic pattern below 3000 feet; or
c) they are transponding on one of the special purpose codes or on one of the other specific conspicuity codes assigned in accordance with the UK SSR Code Allotment Plan.
The AIP is not quite so clear but paras 1.2 and 2.2 of
ENR 1.6.2 explain it.