Snow chains in the UK - the law?

I seem to recall some discussion on using snow chains in the UK some time ago on here but the search isn’t bringing up anything useful. What I am after is the law (if there is one) on how deep the snow needs to be for you to be legally allowed to use snow chains. I’m sure I saw some figures and sources quoted in the original thread. Anyone know? Thanks.

theaa.com/motoring_advice/ov … tyres.html

Doesn’t seem to be a set minimum depth of snow,just says"should be deep enough to avoid damaging the road surface"

OK thanks. Does anyone know of anything OFFICIAL about their use, depth of snow etc?