I have been gone from England for a long time, but they tell me you now have emissions testing. If you can find a friendly tester get him to run an exhaust analysis to see if your engine is running rich.
But looking at the spec of your engine I don't think economy was on your mind when you put it together.
You want high performance, you pay for it
I didn't build the engine or spec the parts used, the chap I bought it off did all that. Don't get me wrong I'm pleased all those modifications are there I just figured I'd get slightly better MPG if only by a few figures.