Let's see now.
United :
Known worldwide
Long history on the LHR-JFK route
Big transatlantic network on other routes
Big marketing operation in both UK and US
Big base of corporate clients in London and New York
... failed on the London to JFK route and pulled out. seen off by AA, BA, VS.
BMI :
Secondary in UK, hardly known in US
No transatlantic history from London
Handful of non-London transatlantic routes
Small marketing operation
Small base of corporate clients in London and none in New York
..... think they can do better.
Hmmm.