The British SWP does not pay its fulltimers the sort of money that would allow them to maintain a staff of highly trained specialists. Neither do any other small left groups.
The British Militant had its arguments on economic policy essentially developed by Andrew Glyn for many years, although they certainly weren’t paying him. Glyn was one of the world’s leading “Marxist economists”, although he would for obvious reasons have hated the notion of a Marxist economist. I doubt if that would have satisfied the “where are the costed budgets” / “the left is losing the argument about economics” brigade however, given that his core arguments were generally about counterposing a socialist transformation of society to capitalist Keynsianism.