You can not work it out without either an MP or Fuel flow meter.
To get close you could ask the prop manufacturer if they can tell you from the blade pitch. We have a Sensenich with a 78in pitch. Ed Zercher at Sensenich gave us an idea of the performance. RPM is useless unless you can factor in other stuff. Most RPM guages are 100-200 out, either way.
Probably the only way for you to get any where close to accurate is just to go to 8500ft on an ISA day, and fly with the throttle wide open (but being in London, that may be tricky!)