There is a 10 page information document released by the DOT on definitions of the "Road Hump"
and it also lists the max advised height.
Here is one section of that document.
Road Humps
5. The maximum height for road humps is 100mm but 75mm is recommended by the DOT. The recommended gradient for the
"on/off' slope of a flat top road hump/raised junction is 1: 15
For the entire script go to http://www.sustrans.org.uk/assets/files/guidelines/traffic%20calming.pdf
And after reading it you can laugh your rocks off.
We all know that this is a load if COD, because I went out and measured the ones in West Barnes Lane in Worcester Park.
The lowest was 10" and the highest was 12.5".