That’s a good thing. Expanded slogans have object-level meanings. Acronyms just stand as identity markers, and are much easier to dismiss.
It’s harder to argue against “Make America Great Again” or “Black Lives Matter”. Their object-level meanings are fairly anodyne and positive. It’s much easier to argue against “MAGA” or “BLM”, since the meaning is obscured by the acronym.
Political movements with nice slogans should avoid turning them into acronyms.
And Make Britain Great Again is so much better than America/Europe/literally anything that doesn't have great in its name