Depends on the baggage situation. Without checked baggage, Int.-Dom. is easily done in under 50 minutes.

With checked baggage, however, it's a lot more difficult. LHR won't tag you on the flight at 13:50, which means you have to get your bags over to T2 to check in. By the time you have retrieved them, cleared Customs, and made it to T2, you will be cutting it extremely fine. Without status, checking in at T2 could take a while.

But why worry? The schedule will change by then, anyway.