Generally the bike and clothing must be either "Old Skool" or permitted for Road Racing under current British Cycling regulations: modern Time Trialling equipment isn't allowed.
So:- No Tribars; no disc wheels or wheel covers; no 3-spoke or 4-spoke wheels; no aero helmets. Maximum wheel rim depth 6cm (six centimetres).
Helmets, if worn, must not cover the ears nor have a pointy tail.