I defer to those who know what they are talking about but I think there
were
at least some basic rules. For example 1-99 were routes that orginated
in
central London, 2xx were once single deckers

I'm not so sure about that. I don't think the 83 has ever gone
anywhere near Central London,

Well perhaps I needed to say mostly or generally, and perhaps the range was
perhaps more like 1 to 80 or thereabouts. As a very rough answer to the
original question pending the arrival of an expert with detailed knowledge I
think I was on the right lines though.

and the 207 wasn't a single decker - it was the old 607 trolley.

Yes and the 259 and the 279 were the 659 and 679 trolleys but trolley bus
replacement was only 40 years or so ago and the 'once' I referred to goes
back before then. The top fifteen or twenty numbers or thereabouts in the
2xx range were once night buses before the N prefix came in.