Sorry if this idea was examed before. But I think it might be necassary to review.

Currently most of the BA's LHR flights are from T5, with few to BCN, MAD, HEL and VIE etc are from T3.

I have nearly missed one flight to VIE due to blindness going to T5...alas...

So how about this idea: Since T5C now will be in fully operational. Few bus gates will be required (even though it was not the case from my last week's travel). So if anyone went to wrong terminal, they could still check in T5 and go to A10 to wait for the scheduled bus to connect to T3 (few of those T3 departed European flights are actually bus gate as well). If you miss the bus you miss the flight.

So as a frequent traveller to BCN or MAD you can check-in in T5 and drop the bag. A scheduled bus will be available 25 minutes before the flight departure to T3 in the airside. So you have a choice of earlier bus to T3 or lounges in T5.

The downside is that the flight would be more difficult to manage and add man power. The good thing is that passenger will have a choice and they will be assured they will be on BA even if they are in the wrong terminal.

(PS: currently the machine in T5 will not print your BP if your flights are from T3).