<brainstorming> Total blue-sky thinking
The nub of the problem seems to be that the second transactions happen "outside the jurisdiction", it becomes very difficult to administer any system to monitor the transaction ...
I'm idly wondering if there is a mechanism to lever the currency - sterling - such that transactions in sterling can be taxed. Wherever they happen in the world.
Not really sure *how*, but it's worth a sideline that until recently, OPEC insisted all oil was bought/sold in US dollars, which acted as a boost for the Federal Reserve.
(Side-sideline, the first country to unilaterally stop using USD was Iraq. Followed by Syria, North Korea, and Iran. Curious how these countries were very much Dubya's "axis of evil" .....)