With WTA International tournaments, like Birmingham at present, there are also 2 top 20 WCs, but if there is no-one to take these then the tournament can allocate one only of these to a lower ranked home player (or anyone else eligible). However if one of the two is taken up by a top 20 player then the other reverts to next player in, and cannot be transferred to a home player. With Premier tournaments like Eastbourne there is no flexibility to allocate the top 20 WCs to lower ranked home players
I have looked around at the usual sites for this kind of information but can't find anything. In any case the draw is tomorrow night so not long to wait.