BA do have a lot of branding inside the terminal (above check-in Zones A and E, for example) and indeed at each entrance above the door (although this is not very visible).
There’s also the welcome roundabout at the end of the M25 spur which features the BA logo.
All of this branding comes at a significant cost, don’t forget. It’s Heathrow’s terminal, not BA’s - whatever they may like to think.