I watched an interesting interview with Sir James Dyson last week. In it he said that no open access to the single market wouldn't be a problem. He implied that soaking up tariff costs shouldn't worry British based businesses. His expectation was they would be set at 2-4% ( no idea if that's realistic but he knows more than me ). He also stated that any lack of open access would be more than made up for better access to the rest of the world.
Sir James seems to have done quite well for himself so I think I'll believe him. Would make hard Brexit worth going for.
We'll see when the demands are set. Donald Trump seems to have done for himself too, but I don't think I'll take his advice. Perhaps the gov of the BoE, or the IMF chief can offer better advice.