I haven't been on duty when we've subsituted the GLA/EDI services, but we did this on a Gatwick - Toulouse service last summer when we were short of aircraft. The catering was standard "All Day Deli" fare throughout and the distribution of pax was as follows:

First Cabin - Closed
Club World (Flat Beds) - Premier, Gold and Silver Cardholders and accompanying families. (This was a commercial decision made by the Duty Officer of the day based on the idea that those travellers sampling New Club World for the first time may well authorise travellers in their businesses to buy it in the future!)
World Traveller Plus - Seated in order of check-in
World Traveller - Seated in order of check-in once all available WT+ seats had been used. (World Traveller pax seated in back rows of aircraft with about 12 empty rows towards the front "blocked" for trim purposes.

For information, the aircraft has two requirements for Weight/Balance purposes (there may be others, Flying Crew, Dispatch staff and those FT'ers out there with a bit of technical knowledge may know)
(a) The critical requirement for Take-Off and Landing (this is why the computer "Departure Control System" will not allow the flight to close until the weight distribution is corrected). The Flying Crew (or George, the Autopilot) can adjust the aeroplane in flight to compensate, but not at the critical take-off and landing stages.

(b) For efficient fuel consumption, most civilian aeroplane designs perform best with a slight nose up attitude. (This is why seat changes are nearly always from front to rear!) As soon as the flight plan has been filed, the Flying Crew calculate their fuel requirements for the trip and advise the Dispatcher of the quantity they need. If the aircraft trim is wrong, the fuel taken on board could be insufficient - again another reason why DCS will not allow the flight to close!