Not all return trips originate in the UK. In particular Monarch were a major provider of scheduled air services to/from Gibraltar (which is after all a British Overseas Territory) and there will be many Gibraltarians who have had to make alternative arrangements for return flights.
Bizarrely, passengers with return flights to the UK originating in Gibraltar can use the replacement services for their outward flight at no additional cost (up to the cutoff date), but must make their own arrangements for the return journey home.