This forced competition stuff is a mess anyway. What happens if the new competitor starts up and culls flights after the shortest of time because they cannot make them work - or do not want them to work because - surprisingly - all of a sudden a certain airline grouping is offering them attractive wet-lease deals or feeder flights to hubs on the continent..... The OFT cannot force an airline to operate on the route or have own slots at its disposal to look continously for new carriers that are willing to take the risk.
Although I must admit that there have been examples in the past. I remember when Lufthansa took over Eurowings, it was forced to prop up small regional independent airline EAE. For examples Lufthansa had to make sure that customers cpuld collect M&M points on EAE flights, Eurowings had to sell five ATR42s at reasonable prices to EAE, Lufthansa/EW had to vacate some routes and hand them over to EAE. But eventually EAE disappeared into insolvency and that was that as far as competion was concerned.