a) Attack the serpent-spawn, even though the helicopters are not really designed for that sort of thing (and I am not sure if they are even armed with anything that would be likely to give serpent-spawn anything other than very momentary pause).
b) be used to evacuate and flee the base, assuming that they have maybe three hours notice, tops, and certainly would not be able to remove everyone living there.
You can assume that they are not getting any orders from the MOD. I've already dealt with them.
Supplementary question inspired by dhampyresa :
In the event of serpent-spawn infesting London and the French using their nuclear submarines to attempt to put a stop to the serpent-spawn before it moves across the Channel, how much of Northern France would become uninhabitable?