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Did a little digging, appears reason is Pilots will start work at LHR and get a taxi to LCY, am not a crew planner, but how about reporting for work at the airport you are flying from.
To save money would guess once pilots get back to LCY they stay local and operate next day, may be called back to back.
If I guess wrong, pilots will taxi back to LHR with all the costs and hours topping up.
Tell me I guessed wrong ????