If that's the case+ and I have no reason to believe it isn't), what reasons could they have for leaving? I have been listening to BBC world news, and whenever

If that's the case+ and I have no reason to believe it isn't), what reasons could they have for leaving? I have been listening to BBC world news, and whenever the subject comes up, the people speaking for an exit don't really seem to put forth a good argument( I hear a lot of 'keep our nationality" and 'we founded parliamentary democracy ' - but they don't seem like a good reason to leave).