If you live in the Essex area there is a free measuring service, if not here are the measuring instructions.
A.- Width from outside of valve to valve, add 150mm to this measurement, remember to check if anything will stop you from adding 150mm along the wall. Maximum width is 2400mm.
B.- Height from floor to the top of the radiator, add at least 50mm to this measurement, check that there is enough room to add 50mm.
C.- Depth from wall to outside of radiator, add 70mm to this measurement, make sure this doesn't intrude too much near a doorway, etc.
D.- Skirting cutouts - Measure height and depth of skirting, allow 2mm around them.