I'm no expert on ATC procedures but would guess at the following:-
Runway 26 in use. Most if not all outbound traffic from the main and north aprons currently routes clockwise via taxiway Echo then Delta. Once Foxtrot is in use it can then turn left and keep the rest of Delta clear for inbound traffic to the east apron, which currently gets held at Alpha 5 until Delta is clear. The current problem of pushing back on to Delta and facing south will remain as it will still block traffic heading via Delta into Foxtrot. Ideally Foxtrot needs to start where Echo ends except that Harrods hangar 202 and stand 62 are there.
Runway 08 in use. Inbound traffic vacating at Alpha 1 currently taxies inbound via Delta, with a few exceptions during quieter periods, mostly destined for the southern half of the main apron. Foxtrot would permit aircraft from the north and east aprons to still head south via Delta while inbounds routed via Foxtrot. El Al, incidentally, will always taxi in and out via Delta regardless of the aircraft size (767's and 777's aren't permitted to taxi west on Echo in any case due to wing tip clearances).