""Possibility"? It was presented to us as a promise. And that's a fact. I saw it on the side of a bloody huge great big bus."

It is carefully overlooked that the wheels on the bus went round and round due to the actions of the https://en.wikipedia.org/wiki/Vote_Leave
Campaign which had cross party support if you study the makeup.
The actual wording on the bus was: We send the EU 350million a week.
Let's fund our NHS instead.
Diane Abbotwas convinced it was Nigel Farage that was responsible butshe appears confused on many issue , does she not.
When the new PM was in power the slogan was promptly dropped.

You may regard it as a promise supported by both conservative and labour, but I would regard it as stating a possible option.
The side of a bus is often used to advertise. Do you regard all their spiel as promises?

It is note worthy that the chair(person) of the leave campaign responsible for the slogan was Gisela Stuart, Labour MP for Birmingham Edgbaston