What law is that Chris?. If you are inside the EU you are automatically part of the Customs Union and Single Market, so I
wouldn't have thought there would need to be any specific EU laws that covered it. I've no idea tbh so happy to be proved wrong.
Article 50, which deals with leaving as you know, is sadly pretty vague. Below is the actual wording specifically relating to negotiating the future relationship. If anything it implies that it should be done in tandem "setting out the arrangements for it's withdrawal, taking account of the framework for it's future relationship"
." A Member State which decides to withdraw shall notify the European Council of its intention. In the light of the guidelines provided by the European Council, the Union shall negotiate and conclude an agreement with that State, setting out the arrangements for its withdrawal, taking account of the framework for its future relationship with the Union. That agreement shall be negotiated in accordance with Article 218 (3) of the Treaty on the Functioning of the European Union. It shall be concluded on behalf of the Union by the Council, acting by a qualified majority, after obtaining the consent of the European Parliament"
Article 218 (3) just deals with who does the negotiating. I've looked unsuccesfully for somewhere that states specifically that the withdrawal agreement has to have been signed and the UK had become a third country, before the future agreement can be negotiated. I'm not saying it doesn't exist, just that I can't find it. But it is a huge stumbling block as I said. I'm sure we would be able to get a withdrawal agreement through if MPs knew what the future relationship would be. There is a distinct lack of trust on all sides and to a degree I can understand this. I wouldn't trust any of them either.