This is straitght from CAP168 -Licensing of Aerodromes, which can be downloaded from the CAA's website -
Civil Aviation Authority Home Page
Chapter 7 - Aerodrome Signals, Signs and Markings
3.2.1.2 All in-use taxiways should be designated by a letter of the alphabet – Alpha, Bravo, Charlie, etc. – however, the letters Oscar, India and X-ray are not used. As far as possible the allocation of designation letters should follow a logical pattern eliminating the possibility for confusion. Where there are more taxiways than letters of the alphabet, double letters should be used to designate short taxiway stubs between a runway and parallel taxiway or between a taxiway and adjacent apron.
As for how holds are named/numbered:
"3.1.3.4 Where a runway is served by more than one Runway Taxi-Holding Position located on the same taxiway, the Runway Taxi-Holding Positions should be numbered in a logical manner." ...
i.e. normally from West to East/North to South along the same taxiway etc...
Hope that helps!
Britkit