Going a bit further back, my 1967 'British Bus Fleets 15 - Midland Red' (when it was sill BMMO, so pre NBC/PTEs) had routes listed 101-994 without any terribly large gaps (more than 10 or 20), following on from X1-X99; and than had A, B, C, D, E, H, K, L, M, N, R, S and W prefixes to 1 or 2...