I remember seeing a site detailing the history of Hants and Dorset some years ago and noted that their lowest-numbered route in the early 80s or so was 30 (the Southampton-West Totton route), with the numbers going right up to 3xx for the Basingstoke area. A lot of the Blue Line route numbers used in the 1990s were inherited from these old numbers, e.g. the 52 to Petersfield or 48 to Fair Oak appeared to be long-standing rather than post-privatisation numbers.
I wonder if starting at 30 was a deliberate plan to avoid route clashes with Citybus in the Southampton area?
These days, by contrast, the main desire when numbering buses appears to be to get the number as low as possible. Difficult to find any three-digit routes these days, though I think (last time I looked, anyway) that the ex-Badgerline network in the Bath area still uses many of the three-digit numbers that were in use back in the 1990s. Not sure why high numbers for bus routes are deemed difficult to understand now when people appeared to make sense of them in the 1980s when they appeared to be commonplace... for instance, Alder Valley, London Country and Southdown services in the Chichester-Guildford general area _all_ seemed to be three-digit.
To be fair, thankfully I haven't seen duplicates from the same operator (until recently, we had two 1s in Southampton but one is Bluestar and one was First - now gone) though you do get some things like Stagecoach Hampshire having different route 1s for Winchester, Basingstoke and Andover. Probably won't cause confusion as they never run in the same town but even still, it seems conceptually 'neater' to have one operator (e.g. a local division of Stagecoach, etc) use unique numbers across its network.
Actually, if you look at the pattern here (in Hampshire) there has been a progressive lowering of numbers since privatisation. The first step was to eliminate the three digit numbers, so Stagecoach Hampshire for instance generally ran buses in the range 1-99. For a while, IIRC the Basingstoke services got numbers between about 50-70 for instance. Fair enough as it was a smaller company/division than the old Hampshire Bus/Hants and Dorset. More recently, though, even those numbers seem to be too high: the fashion on town services for the past 20 years or so definitely seems to be avoid going above about 10 where possible!