On one ticket, UA will be responsible for rebooking you if they blow your arrival at LHR. Doesn't matter that EI isn't *A.
The 3.5 hour connection gives you lots of time, obviously.
LHR has a nifty "Connections" function on its website, with step-by-step instructions on how to transfer:
heathrow-airport-guide/flight-connections.
Unfortunately, no info on T2 yet, but I assume it will be updated prior to the terminal opening.