A very interesting question KerryMLC!

You are correct in that Pier 7 at T3 is the only significant part of LHR to be horizontally segregated between arrivals and departures rather than vertically segregated.

Full segregation is maintained at all times, although this can be operationally challenging as passengers may have to wait to proceed down the corridor (either for arriving or departing flights) if a cross-corridor flow is in operation.

The gate numbering isn't quite how your diagram portrays it: Gates 16, 17, 19, 21 are on the south (departures) side of the pier, whilst Gates 13, 18, 20, 22 are on the north (arrivals ) side of the pier.

Gates 14 and 15 don't exist - this area is the Virgin Atlantic Crew Report Centre.

Cross-corridor doors are available for all gates to allow both arrivals and departures with the exception of Gate 13 which is for departing flights only. Aircraft are towed to this stand before departure (because they can't turnaround from an arrival at the same gate).