The UK trade restrictions only applied to the Eurozone. They always had the ability to form their own trade relationships outside of Europe. Brexit wasnt needed to make deals in Africa.
Sorry, but that’s not the case. Trade deals with non-EU countries are negotiated and agreed by the EU as a whole, not by the individual member states. The treaty rules oblige member states to abide by this - if they didn’t, it would make nonsense of, for instance, the EU customs union. In consequence, throughout the years of EU membership the UK has had no trade negotiators of its own (although there have been British trade negotiators working for the EU), and has had to recruit trained and skilled negotiators from elsewhere (particularly Canada and Australia) to gear up for the freedom to negotiate post-Brexit.