BD departs from Terminal 1, so you do not have the inconvenience of changing terminals.
The walk from the UA gates (e.g., 146) to the immigration line takes at least eleven minutes - I do it frequently - (longer should the two travelators not be working). There are several steep slopes to negotiate from the jetway to the upper level walkway. The delay to pass through immigration is usually less than five minutes. There is a separate line for transit passengers (on the left-hand side).
The Star Alliance lounge is not very well signposted, the entrance being almost hidden, but once inside there is hot food and a good range of sandwiches (something which RCC needs to provide) and plenty of seating. The windows of the lounge do not give a view of aircraft.
I hope that you enjoy your brief stay in London.