It will still be tight! The security regulations of the U.S. force LX
to have their flights to the States leave from another concourse than their European flights arrive (Long xray screening process, take off shoes and all that). If the LHR
flight has a slight delay, prepare for a night in a good hotel (unless the LAX
flight is waiting for many connecting passengers). Then the bag connection's at risk...
If the LHR
flight is on BA
, then it's no problem as BA
arrives in the same concourse as the U.S. bound flights leave.