The most common Gauge used by modellers of British Railways is 'OO Gauge', which shares the same track as 'HO Gauge'. While both these 'Gauges' run on the same track, they are actually different scales, with 'OO Gauge' being 1/76 Scale and 'HO Gauge' being 1/87 scale.
As both scales use the same gauge of track, this means one has to be inaccurate - and this is OO Gauge. OO Gauge track is technically the right Gauge for HO rather than OO, and this has led people to model 'P4' and 'EM' Gauge, as that is actually the right Gauge of track for the scale. Keep in mind though that the different between OO and HO is not too noticeable, and that it is very much common practise for British OO modellers to just accept that the track gauge is slightly off!
To put it as simply as possible:
OO Gauge - scale of 1:87 or 3.5mm to the foot for track and a scale of 1:76 or 4mm to the foot for the locomotive bodies.
HO Gauge - model trains AND track with a scale of 1:87 or 3.5mm to the foot.
This doesn't mean that HO Gauge British models do not exist, these in fact used to be produced by Italian manufacturer Lima, but never gained as much popularity as Hornby's OO Gauge models, and all major British manufacturers continue to produce OO Gauge models to this day.
Popular manufacturers of British OO Gauge models include Hornby, Bachmann trains, Dapol, Heljan and Rapido Trains, with many new manufacturers recently joining the scene including Cavalex and Accurascale, who even produce models with the ability to swap wheels so locos can run on the more accurate EM and P4 gauges.