UA couldn't make the UK regional feeds to EWR work, in part, because they always priced them at a noticeable premium compared to LHR
In fairness Continental did well doing just that, but remember Continental didn't have access to LHR before 2008. Once they were in the door, there is an arguement to max out profitability on expensive LHR slots and feed the BRS/BHX traffic that way. Once United took over, an airline with no history in regional UK flying outwith a short lived GLA-IAD, the decision get's easier IMHO.