This is a Marxist analysis of how the financial system supports the appropriation of surplus value from other countries. While the focus is on Britain, the analysis shows how a Marxist understanding of the international financial system can throw light on a key aspect of the economics of imperialism today, one that follows from the normal, day-to-day operations of financial markets in the imperialist world economy. It argues that the economic content of imperialism is not only based on the trend towards monopoly, but on a world economy dominated by a small number of powers, especially in the sphere of finance. Marx's concept of fictitious capital is found to be superior to Hilferding's concept of finance capital for analysing these developments. However, the thesis also shows that 'fictitious' deposit creation by banks, largely ignored by Marxist analyses, is a key part of the financial mechanism. The thesis demonstrates the key role of finance for British imperialism through discussing the evolution of the global financial markets since 1945 and analysing the structure of the UK balance of payments data over the past three decades. It shows the importance of revenues from financial services exports and the way in which banking and portfolio flows fund relatively high returns from foreign direct investments. This analysis also shows how the UK-based banking system intermediates global financial flows, providing finance from developing countries to other major powers, particularly the US, a feature that is closely linked to the role of UK-linked tax havens.