Among the more impressive ones, have a look at table C3.2 that details External business of monetary financial institutions operating in the UK. That one totals $23,508,170 million ($23 trillion).
There's also a very interesting Bank of England report that you can find here. It's the BIS Triennial Survey of Foreign Exchange and Over-the-counter interest rate derivatives in April 2010. Apparently, they only do this once every three years. It says "Net average daily turnover during April 2010 in the UK foreign exchange market was $1,854 billion per day". I guess that means we can multiply the figure by 250 working days to make something like $463 trillion over a full year.
Then we learn that "In the UK, average daily turnover in OTC interest rate derivatives increased ... to $1,235 billion. That's another $309 trillion a year.
Finally, here's another Bank of England table you can download here. Table 6 shows Global Over the Counter interest rate derivatives to be running at $2,083 billion per day - about 521 trillion in a year. But it also slips in a figure for Exchange-traded derivatives of $8,142 billion per day - over $2000 trillion for a year.
A 1% transaction tax on those too would already generate quite a lot for the UK government. No alternative to increasing VAT to 20%? No alternative to slashing public services? No alternative to making hundreds of thousands of people redundant and cutting pensions?
If anyone out there wants to help compile the complete set of figures for financial transactions in the UK, I would be very happy. Just add a comment, and I'll do my best to add up the numbers.