The single market?

I think that unfortunately because they are members of the single market', they could not operate unilaterally, though maybe they wish they could. We can trade unilaterally with any other country in the world once free from EU and will be able to bid for trade that is specifically in the interests of UK and not the 27others.
I always likened the single market to living on a housing estate, where you had to ask permission to buy any furniture, ideally you would buy from a neighbour. If not the neighbours set how much you could spend , what colour it must be and before the actual purchase would have to view it and everyone would have to say yes it looks okay.

It is not just our imports/exports TO and FROM the single market. Being in it also affects how we trade with the rest of the world - the deals that the single market has made with other countries apply applied to us. We got the same terms as everyone else in the single market for our imports/exports to them. all the agreements will have to be redone. Some hard work lies ahead horse-trading our smaller (but perfectly formed!) buying public against the bigger sales that the single market can offer.