1) The UK could insist that sovereign funds looking to set up shop in London meet a high standards for disclosure. If the forecasts from banks like Merill Lynch are to be believed, sovereign funds may soon be adding $1 trillion a year to their assets. At that pace, to paraphrase a Ken Rogoff quip, sovereign funds quickly will become the global financial system. Even if those forecasts don't pan out, some black boxes look set to get big fast. A lot of them seem to have large operations in the UK. Without a bit more (retroactive) disclosure of the broad contours of their portfolios (of the kind in the IMF COFER data), it will be hard to assess their contribution to any future "underpricing of risk."
2) Upgrade the UK's balance of payments statistics to match the US statistics. Specifically, the UK could provide a breakdown of the geographic origin of inflows to the UK and the official/ private split. As more and more global flows move through London, the absence of more detailed data increasingly impedes real time and historical analysis of global capital flows.
The UK's data is here. Best I can tell, even in their comprehensive annual publication, the UK only provides a geographic breakdown for the current account, not for the financial account.
-If the UK wants to increase financial transparency ...