Is there any reason why BA could not do a front and rear boarding at more airports? I presume there are additional costs associated with it, but would have thought that these might be offset by a better on time performance.

As regards the holding area, these do not necessarily have to be unpleasant. KLM uses this method for some flights out of Schiphol, but the holding area is usually pretty big, and the scanning starts only a few minutes before boarding, so your time in the holding area tends to be minimal. I've also experienced it at LCY, where it was very cramped.