How early is early? I think UA have 2 flights going our between 7.30 and 8 am?
Personally I find the Holiday Inns fine (Heathrow or Heathrow Ariel) with 24 hour free bus service ('red' london buses are free on the airport perimeter).
Hilton or Sofitel are normally very much more expensive options, but you can use the T5 or T4 to Heathrow Central (For T1 and T3) train transfer, both every 15 minutes from 5.07am from T5 or 5.23am from T4 (monday-saturday). The first train from T4 on a sunday is a bit later 5.53am.