We get excellent visibility just as the warm sector starts arriving, under the slowly lowering veil of frontal cloud. There's a saying at Sennen, if you can see Scilly it's going to rain.
Also in 'foggy' warm close to high centre. That me seem strange, but out of the banks of sea fog the visibility is normally excellent. You can look across Mount's Bay at various banks of fog, some sea fog banks, more associated with uplift on the SW facing headlands, and at the same time see the LIzard Point so clearly it's as if you can touch them, the excellent visibility shrinking distance so. (The best conditions for getting seriously sunburnt.)
A particularly dramatic picture to illustrate the point, not mine though I discussed at the time. John Chappell ( http://www.landsendweather.info/
) and I both being confident the the dramatic feature was associated with the onset of the north coast sea breeze rolling past Lands End. Picture looking west, fog bank moving south.