Agree with you there, 411A.
Perf A requirements may well require that an a/c should be capable of continuing from V1 to destination with 1 engine inop; however, whether it would be desirable to do so is entirely another matter.
Not sure what the actual route from LAX to LHR would be, but the Great Circle is over Hudson Bay and Greenland. A lot of cold, inhospitable areas to cross with 1 engine out, in my view. A diversion to a suitable CONUS aerodrome for maintenance would seem rather more sensible than to carry on all the way across the GCFA and North Atlantic on 3?
And I'm not sure that I'd like any 'commercial advantage' to feature in the airmanship plans of anyone who makes that a consideration before electing to cross the pond on 3... Whether in a 747, 707, Comet or DC-7 for that matter.