http://www.caa.co.uk/docs/1859/20090...idanceV6-1.pdf
It's in the section titled 'UK Specific Limitations'.
The three classes of medical were, as far as I can remember;
One - for public transport,
Two - for aerial work,
And Three - for PPL
When these were changed to the two classes that we have now, some pilots flying on an original Class Two didn't meet the standards of the New Class One required and so were issued with a Class One with the limitation as described in the above document, commonly referred to as a 'Restricted Class One'.