In article , Henry writes
Is there a logic to the numbering of bus routes in the London area, such a
1-100 means one thing, 101-200 means something else, or are they random,
maybe loosely based upon a logical system of long ago that has fallen into
disuse?

Pulling together various comments here and elsewhere.

1-199: original Central London routes, later "red buses".

2xx: originally used for single-decker routes, then for trolleybus
replacement, then merged into a single 1-299 block.

3xx: originally "green buses" (London Country) north of the Thames.

4xx: originally "green buses" (London Country) south of the Thames.

5xx: trolleybuses, later reused for Red Arrow

6xx: trolleybuses

7xx: Green Line

8xx: various special services

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