How are the numbers of UK buses assigned?

Are the numbers related to the route the buses take or could they possibly have any chronological significance??How are the numbers of UK buses assigned?
For London it is simple.





The Road Traffic Act (London) 1924 imposed a system of bus route numbering on all operators entering the metropolis. The actual numbering was left to the Metropolitan Police, and, in particular to Acting Chief Constable Bassom.





The system that evolved was called the Bassom system. It was un-necessarily complicated and was offically abandoned in 1933 when the London Passenger Transport Board was formed.





Bassom divided routes thus:


1-199 Double deck central area routes


201-299 Single deck central area routes


301-399 Country area routes north of the Thames


401-499 Country area routes south of the Thames


501-529 Country area routes in the Uxbridge area


601-699 Trolleybus routes





When the system was abandoned, the LPTB kept most of the routes, although there was a degree of simplification. The single deck/double deck divide was given up as a wartime economy measure when service 253 was converted to double deck operation without a change of number.





Post-war saw the Green Line services numbered in the 701-727 series and the 800 series used for Country area services in some New Towns.





Over 75% of todays route numbers used in London can be traced back to Bassom.


TheHow are the numbers of UK buses assigned?
Here in Bristol, the numbers 1-99 seem to be ones which stay within Bristol, whereas ones from 301-399 go out to nearby towns and villages which are not part of Bristol City Council. You will also find that buses taking broadly similar routes but with minor changes at the beginning or end will often be numbered similarly, or they will have an 'A' stuck on the end. Any buses which have an X in front of the name (e.g. X32) are express buses going slightly further afield (e.g. Bath) with a limited stop. The other rules don't seem to apply to them though.
Is this question about buses in London only? The numbers are not really related to the route which they take. Every time a new route is added, thats the number the route has, for example, Bus 186, would have been the 186th route to be added in London... however this was the old system..





Night buses have an 'N' in front of them, eg N98





Nowadays its all very random, you get routes like 34A, so the system has gone a bit out of whack, but if you're looking to get around by bus in London., I suggest journeyplanner.org
Historically, bus routes run by London Transport were grouped by the type of service they provided -


1-199 - Inner London red double-decker services


200-299 - Inner London red single-decker services


300-499 - Rural services


500-699 - Tram/ Trolleybus


This no longer applies
The answer about Bassom is spot-on, although Ken Livingstone messed about with so much of the London Bus network, much of the historical links have been set aside.