The marginalist revolution posed a fundamental problem for economic theory. Mises’s approach not only solved the problem of economizing resources in a division of labor, but provides a robust framework for economic research.
London is a major global hub, and a hard Brexit wouldn't change that. But one of the best possible outcomes of Brexit could be a move toward real free trade beyond the faux "free trade" of the EU bloc.
MMT basically holds that governments have control of unlimited amounts of real wealth — thanks to money-printing power. But if this were really true, countries like the USSR and North Korea could simply create money until they became wealthy nations.