What serious manpower shortage at LHR are you talking about? BA was overmanned due to its reduced flying programme over a two-year period which led to 16 planes grounded last winter - though two of the eight 747-400s have been brought back from the Mojave desert, one for DFW and the other for LAS on 01JUN11.
Also, the only permanent recruitment in recent times before MF has been at LGW. If BA needed more cabin crew at LHR, it would have been quite easy to recruit cheaply at LGW, thus enabling some of those waiting impatiently on the transfer list to move to LHR.