If sincere, they must apply this to every seat, not only those falling vacant, so they should be preparing all constituencies for redistribution. After all, the next election may not be too distant.
This might assume a three-way division into equal male, female and ethnic minority aspirants, but there are obvious subdivisions. Assuming that males and females would be "non-ethnic", there would be a clear case for separate and equal male and female minority candidates.
I would suggest a further priority category: given that the coalition Cabinet started out with the top four positions – among others – occupied by multi-millionaires, let's have a proportional measure of members with no outside income.
Tranent, East Lothian