I'll definitely have a go.Both United Automobile and West Yorkshire Road Car had alpha numeric systems until the 1960s when new fangled computers meant they moved to purely numeric. West Yorkshire, for instance, had a SUG (Single Underfloor Gardner) or CRG (Coach Rear Gardner).
Eastern Scottish also had an alpha numeric system, plus an end letter to denote allocation. I'll let @DunsBus have a go at explaining them all!
Single Deckers
A - Bristol
B - AEC
C - Bedford
D - Daimler
H - Leyland (until 1984)
L - Leyland (from 1984)
M - MCW Metroliner
O - Optare MetroRider, also the sole Omni
R - Renault minibuses
S - Seddon
V - Volvo
Double Deckers
AA - Bristol Lodekka
DD - Daimler/Leyland Fleetline
EE - the four Dennis Dominators which were in the fleet for a brief spell in 1986
HH - Leyland Olympian (until 1984)
LL - Leyland Olympian/Lion (from 1984)
MM - MCW Metroliner
VV - Volvo Ailsa/Citybus
Prefixes
C - Citylink coach
M - Minibus
X - Toilet coach
Y - Touring coach
Z - Dual purpose
This could throw up some interesting combinations. ZS was a dual-purpose Seddon, YL a coach Tiger, but then you had this - XCMM denoted an MCW Metroliner double decker coach, with toilet, used on Citylink work.
The prefixes were dropped in 1994, with the type codes following suit the following year.
Depots
A - Edinburgh (New Street) and Biggar outstation
B - Bathgate
C - Baillieston
D - Galashiels
E - Kelso
F - Linlithgow
G - Dalkeith
H - Airdrie
J - Berwick-upon-Tweed
K - Peebles
L - Hawick and Jedburgh outstation
N - Livingston
S - Dunbar and North Berwick outstation
W - Musselburgh
I'm aware that Airdrie had two depots at one time, one of which was inherited from Baxter's and coded V - Airdrie Victoria. Also, very late in the day, Bathgate was replaced by an additional depot in Livingston - Centrex, which was coded Cx.
(Any additions and corrections happily received, as I've compiled this from memory.)

