Safety sense SSL01 - good airmanship. Of course it is not law.
"UNLESS you have an IMC or instrument rating a suitably equipted aircraft and are in current practice and expereinced you must remain in sight of the surface. Before encountering lowering cloudbase or visibility make plans to retreat or divert. If conditions get worse eg 1,000 foot cloudbase or visibility less than 3 K carry out these plans immediately".
It is almost a quote but I couldnt cut and paste from the pdf.
Note the wording that would suggest to me you do certain things if you are not instrument rated but there may be other possibilities if you are subject to the caveats.
The whole section is under the title "diversions".
Whilst we are talking about the CAA it occurs that they could have said "even if you have an IMC or IR if you encounter these conditions and had set out VMC you should carry out these plans immediately (viz turn back) and not risk entering the clouds. (sic because it would be illegal and dangerous to do so).