In a press release (link below) from SGX they say:
Clearing of financial derivatives
New OTC interest rate swap transactions cleared was S$8.1 billion notional in June, up 70% year-on-year.
Since the start of IRS clearing in November 2010, SGX has cleared a total S$273 billion notional of IRS.
Converting to GBP at 2:1 means £4bn per month cleared.