The base closures take effect at the same time the LGW routes cease so time wise it will only be the EXT, BHD, SOU, JER and LIG routes affected.
All of which are achievable. For those not operating from such core bases such as BHD and SOU, a possible pattern could be:
EXT-NCL-JER-NCL-EXT
SOU-LIG-NCL-LIG-SOU-NCL-SOU
Just because the base is closing does not mean the routes will neccesarily stop. However alot depends on whether they actually make any money