It's pretty clear that the banks have all received the same information regarding Farage & made the same decision. Nothing to do with collusion I would suggest. Obviously we don't know what that information is but it is suggested that it relates to illegal payments received from Russia.
I would suggest that all the banks would face the same risk of some sort of potential prosecution if they operated accounts for Farage & thus all decided not to offer him one. Not unlike for example allowing customers to transact with Iran, North Korea etc
