The UK assignments of SSR codes are documented in the UK IAIP at
NATS | AIS - Home The relevent part of it is the En Route Information ENR 1.6
EG_ENR_1_6 paragraphs 2.2.2 through 2.6
Re. 1177:
Use: London AC (Swanwick) FIS
Remarks: Aircraft in receipt of a Basic Service from London Information. Shall only be selected with ATC direction.
It tells other controllers with which ATS unit the aircraft is communicating.