I am all for this change as long as it is enforced as I have witnessed the awful boarding process at LHR too many times. Maybe all those people criticising BA's new policy could reflect on the fact that it is not too different from the often unenforced policy of today and present some ideas of their own?
Given that:
a) there is not always room for all carry-on bags to fit in the cabin
b) priority boarding is clearly something that people value/aspire to
c) passengers could have spent anywhere between £300 and £4000 for a seat on the same plane (long haul/different cabins)
d) some passengers spend huge sums with BA every year and could reasonably expect their loyalty to be reflected over and above less regular passengers
With this in mind, how should it be done?