Generally, I'd surmise they picked an early 200 series number because of the RNAS/RAF (Joint) connection.
https://ukdefencejournal.org.uk/new-...on-become-ocu/
Specifically why 207, maybe as it was a bomber (strike) sqn as opposed to an army co-op/fighter recce (fighter) sqn like 208? All the others being Coastal Command up to 211?