The only other airline that could make the remedy slots work and be effective against IAG on internal UK flights into LHR would be easy jet, although they may have to tweak their baggage allowance to take self connecting traffic even then i would suspect it would do little more than break even unless they could get a deals with non IAG airlines at LHR, the application of APD to connecting long haul flights add ££'s to ticket costs that you can avoid by self connecting at say AMS