The LGW crew can report at LGW because they are "in hours" to position to LCY and then operate - their Duty Hours calculation will start from report time at LGW, their Flying Duty Hours calculation will start from arrival at LCY.
If the positioning is to be immediately followed by a flying duty then both the Flying Duty Period and duty time start at report for the positioning sector. However, the positioning does not count as a sector for determining the maximum allowable FDP so if they do LGW-LCY-SNN-JFK the maximum FDP will be that for a 2 sector day rather than 3 - for an 0800-1259 start 14:15 instead of 13:30. If positioning after a flight then that would be counted as a duty but not included as part of the FDP.
