Signage at the point you entered
The council must sign the bus lane at the point where you entered. If the sign was missing, defaced, obscured by foliage, or contradicted a nearby sign, the lane is not lawfully enforceable for that approach. Photograph the signage from a driver's viewpoint as soon as you can.
Forced entry by another road user
Adjudicators routinely accept that a driver who entered a bus lane to avoid a collision, an obstruction, or an emergency vehicle was acting reasonably. The legal phrase is "no realistic alternative" — argue the facts, not the emotion.
Operating hours and exemptions
Bus lanes have time-limited operating hours (e.g. 07:00-10:00 Mon-Fri). If you entered outside the restricted hours, the contravention did not occur — that is a category-1 ground. Taxis, motorbikes and bicycles may also be exempt under the Traffic Regulation Order; check the order text not just the sign.