Did T4 Madrid not also have signage problems? (initially)

As for the one terminal for all BA flights thing, you need to remember the time span for the development of this terminal.

Personally I'd have shifted the entire thing outwards more (i.e. towards the M25 direction, away from T1,2,3) to make more room inside and possibly also more gates with increased space between 5 and the central area.
How you may ask? well by losing the connected car park to another location elsewhere connected by a fast transit train.