You could almost certainly negotiate the rate down with Square. Maybe not for £6200/month, but at a certain volume they'll do you a deal. There's no way Philz Coffee (big coffee chain in the bay), for example, are paying Square their advertised flat rate.
Interestingly Square seems more expensive than iZettle. Square is flat 1.75% in-person 2.5% online while iZettle is sliding 2.75%-1%. The threshold seems to be £6200/month at which point iZettle is going below 1.75%. That's pretty low so I suspect most shops would still chose them.