10 Provisional Brigades - serious invasion threat?

[Keelagher]

I have only recently heard of these 10 provisional brigades, all seem to based around coastal places or ports. (My Grandad seems to have been with a Scottish Brigade at or near Deal in Kent). Were the authorities seriously worried about an invasion or were they placed there to be a handy supply of medical aid and even re-enforcements? Did they help with training or loading of ships? otherwise It seems like a lot of men hanging around. (would over 30,000 be an exaggeration)

If the answer seems obvious pardon my ignorance.