O-D, if you'ff forgive me, the prevalence of low-numbered squadrons (and some 200-series numberplates) is a consequence of policy, not direct policy.
The over-riding consideration is seniority, with role assocation as another key factor (thus were, say, a Poseidon OCU to be formed, although the 19 Squadron numberplate is senior, it would be less likely to be chosen than that of 42 Squadron - association via Nimrod and being an OCU for MPA - or 22 Squadron - a pretty senior numberplate, operated Vildebeests, Beauforts and Beaufighters in the ASuW role in WW2).
207 was one of the squadrons which Trenchard selected to be the backbone of the RAF in the much-reduced circumstances of the immediate post- First World War era, and despite its disappearance from the lists after the retirement of the Devons, its accumulated seniority was still sufficient for it to be in the running for revival.
Trenchard required his staff to produce a list of 32 squadrons which had distinguished themselves; the squadrons were to include RNAS units and to represent all roles and all theatres of war. The resultant list had three alterations made to it by Trenchard - 8, 186 and 201 were replaced from that list by 203, 208 and 210. When two more squadrons were required, 8 and 151 were proposed; Trenchard concurred with 8 but decreed that 45 should be the next one. A similar process occurred at the commencement of further RAF expansion, thus the order in which squadrons formed was not on a numerical basis, and seniority and low numbering do not automatically fit together.
Clearly, some of what should be the most senior numberplates around disappeared from the lists over time - for example 267 Squadron was one of Trenchard's originals, but after the retirement of the Argosy and the subsequent reduction of the transport force during the 1970s, no opportunity to reform it has arisen - role association playing a key part in the disappearance. 205, 210 and 238 have also suffered similarly from that 1919 list signed off by Trenchard.
Forgive the diversion into slightly arcane history.