It is not that LHR don't trust any one elses security it is the UK Government and LHR are simply doing what they have been told to do. There are some exceptions e.g. the Little Red flights have a bus from T2 to T3 and those pax don't undergo the security check)
It could be worse and we had the US system - there are minimal flight connections in the same way as here and everyone has to clear the border, collect bags etc.
At least at LHR if you are just in transit you generally don't have to clear the UK border or collect your bags and recheck them in like in the US