There are many codes allocated to units across the UK, some of which are 'conspicuity codes' (they are not all 0260). As well as the explanation given by Friio4, the instruction to select a conspicuity code (which may be allocated to a non-SSR or radar equipped unit such as Cranfield Approach or Cambridge Approach) can allow neighbouring radar units to know who to call if they want to know about something that traffic is doing.