I believe CAA do not permit braking action figures to be passed to pilots, so if the readings are below what is considered to be "good" braking action then that portion of the runway should be published as "slippery when wet" with operators applying their procedures as appropriate. As Manchester have published a Notam that part of their runway is slippery when wet then presumably this is not related to freezing conditions but when the surface is wet with either de-icer or rain. So maybe the Manchester issue is the general state of the runway surface which doesn't provide a "good" braking action when wet which would be a different sutuation to Gatwick's runway the other night which lost braking action due to ice.