A very rough estimate of volume is 3 x 3.5 x 3m, which gives 31.5m3, well
within the limits and enough left over for a lean-to over the side of the
kitchen. The house was built in 1909 and there has been no previous
extensions, just a garden shed, not attached in excess of 5m away from
where
the conservatory would end.

Apologies I have misread it! I thought the 10% was a lower figure for
terraced properties. I also assume your house is not in a conservation area.

AFAIUI Building regs will apply since it is an extension. The best thing is
to talk to BCO before you do the work since it can be incredibly expensive
to correct. Don't forget the new rules on insulation standards if you are
DIYing the conservatory. You WILL need approval for that unless it is
installed by a FENSTRA registree.