A foreign company employing people in the UK has to comply with UK labour law. What you may be saying is that a foreign company which wins a contract in the UK can bring its own workforce with it under its own terms and conditions, and pay them under their domestic law. First, this will not be stopped by ending free movement, only by ending free trade. Second, it works both ways, British companies can take their own workforce with them to fulfil contracts abroad. But this is irrelevant to the situation where the end user is a domestic company. It isn't that a domestic company is contracting a Hungarian one to pick fruit on its farms. It is employing Hungarian workers at UK rates, probably because Hungarian workers are just more productive.