This will get moved out of the UA forum, but before it does:
There's a Hilton near T5, but that's nowhere near where UA departs. All of the other hotels are along Bath road on the north side of the airport where the access tunnel goes under the runways to T1/T2/T3 (United operates from T2). Many are within the free bus zone where you can catch any London bus within the zone without paying a fare. Closest is likely the LHR Renaissance, which is a huge hotel with tiny rooms. Many others along the road there--google maps is your friend to find the chain of your choice.