I am trying hard to understand the rationale behind this. I'm not sure how the EU in and of itself supports any of the creative arts, unless they are talking arts grants, and they could easily be duplicated by an independent Britain, and probably much easier and cheaper.
By definition creativity cannot be regulated, so I don't see how the flow of ideas between Britain and Europe would be affected by an EU withdrawal. By any objective measure Britain is a net culture and creativity exporter. The creative input we get from from Europe is not very extensive at all - it is certainly dwarfed by what we get from America.
In short, I suspect these 300 "artists" are just fabricating a threat to their industry to push their own political agenda.
The EU dumps big bucks in the arts. They do this to buy the narrative that the EU is vital to the member nations. The arts are their Ministry of Truth.