The free trade policy of the EU:
Many pro-Brexit advocates seem to argue that a common European trade policy is a problem for England. It is true, that the UK can´t negotiate their own FTAs with foreign countries, but I don´t really see the problem.
The EU is actually pretty good at reducing tariff barriers with other countries. It is not perfect, but there are only few countries in the world that are actually better than the EU in this regard. The EU is sometimes seen as fortress towards the rest of the world, but that has little to do with “classic” protectionism. That is almost exclusively down to the very high level of standards and regulation. Now I am a fairly extreme liberal, so I´d agree that the EU is regulating too much stuff. Still, even I acknowledge that any developed country has to set standards and while the EU might overdo it in many ways, it is more of a structural problem and has less to do with the EU. Other developed countries have more or less the same problem.
So if you argue, that leaving the EU allows England to negotiate substantially better trade deals (or substantially different trade deals), you have to hold fairly radical views. Either you are in favor of strong protectionism or you have to fancy the radical reduction of regulation and standards; abolishing most labour standards; abolishing environmental standards; reducing health and risk regulations et.al. Some Tory politicians might fancy such a course (and I personally like at least some of these ideas), but I doubt that most pro-brexit voters would actually support such policy. Just changing tariff barriers will make little to no difference, because they are simply not the biggest barriers anymore (with few exceptions) for imports into the EU.
Furthermore many developing countries close their markets to developed countries. That is actually a big problem, but leaving the EU won´t solve that. To the contrary it will put England in a much weaker position. In comparison: The EU is better in securing access to foreign markets than the USA.