Interesting.
We're no longer allowed to quote friction values as it is now regarded that there is no direct correlation between friction values derived from the current measuring devices and braking action. ALL UK SNOWTAM should now be annotated 99. Looking through those airfields that actually bother to publish SNOWTAM these days, most are either blank or showing 99.
At LGW, if we need to carry out treatment on the runway in adverse wx, we work until we can open it as 'wet, wet, wet' or better.
Cheers,
TheOddOne