Excuse my ignorance I did not realize the UK had market makers? I thought their market was electronic with strict Time price priority.
It may reduce your trade deficit by making imports more expensive. You have to buy American for it to have no effect.
http://www.opsi.gov.uk/si/si1992/Uksi_19923286_en_1.htm Seems European Institutions engaging in business in the UK are also exempt. I suppose the Hedge funds trade CFD's with their prime broker or a bank. That is Bank buys stock and sell CFD to the fund. Stupid rarely, but I suppose HM Treasury makes too much cash from the Tax