The Chiltern Line to Paddington is full with no additional paths in the peaks, so would be used only in extreme emergency - and until the single line Ruislip/Paddington is dualled, that can only cope with 2tph each way at a max in any case.
... and the whole route from South Ruislip to Old Oak Common is threatened by
HS2▸ surface work (vent/access shafts etc.) even though HS2 will be tunnelled beneath it.