Bright-ling:
Careful, the 0027 squawk is ONLY allocated by London FIS to traffic to which they are talking and which is requesting a join or cross of controlled airspace. It is used to identify that particular 'maggot' from all the other 7000s out there when they ring the relevant sector and ask for joining clearance.
0027 is not for general use without being asked to put it on, otherwise it loses its effectiveness.