LHR transfers are notorious with queues on certain days or at certain times. Just have plan B ready if you miss the connections. Several of my American friends just refuse to transit via LHR to Europe now after really bad experiences. The delays are to do with queues for security. Transit passengers must re-clear security unlike many American airports.

LHR also have a conformance requirement. You must clear a control point 35 minutes (I think) just before airport security before your for flight even if you are in transit. This is known as conformance. It is not a matter of "running to the gate" for a short connection. If you get to the conformance check point past this time, you will not be allowed to go any further no matter how much pleading you do. You will have to go back and queue at the connections desk to be re-booked.

Good luck.