You apply for a new uk passport and vfs send of all the docs.
You are told you cannot use your old passport for travel.
( At the moment that seems to be a good few weeks)
An emergency arises in uk and you need to go there.
How do you get there with your old passport.?
Decided on an edit.
Who actually says 'you cannot travel', VFS or UK passport office.
How would anyone know if you went to Vietnam or Malaysia say for a few days?