Just to add some detail here since I suspect it's relevant to what happened.

The actual scheduled time of arrival of BOS-LHR BA202 into T5 was 09:10
The scheduled departure time LHR-OSL from T3, BA760 was 10:45
so just 5 minutes over MCT at 95 minutes

The actual arrival time of BOS-LHR was 09:34, and the actual departure time for LHR - OSL was 10:52, so 7 minutes over.
so 24 minutes late, and well below MCT at 71 minutes. They did well really today, the aircraft only had touchdown in LHR at 09:27, so everything went like clock-work. On the basis of LHR-OSL supposedly closing up 20 minutes to departure (hence my comment about being at the gate 25 minutes to departure) that gives about 50 minutes to get from T5, through the airbridge and terminal building to Flight Connections, on a 12 minute bus ride, through security in T3 and then perhaps a long walk to the Oslo gate. It was actually gate 11, which is gate very near the BA lounge, you can see it operating from from the dining area of Flounge. The OP got from T5 to T3 airside in about 30 minutes, which is good going but not exceptionally so.

I guess it's infuriating that even when LHR works efficiently, the robots aren't able to adapt. At 1 hour 11 minutes from T5B to T3 - that's a pretty tight connection, albeit one that was do-able.