This anticyclonic gloom usually occurs earlier in the summer when warm subtropical airmasses over-ride the still coolish seas, leading to condensation in the form of sea fog. If there is just enough wind and you have a strong inversion then this fog will lift but gets trapped under the highly stable inversion. Unless winds change or you get a good break in the cloud deck overland to start off sufficient convection to overcome this cap then these benign conditions can last for days.
Dewpoints have been in the high teens lately so we do have that very moist airmass in place. Looking at the soundings, such as yesterdays
Camborne 00Z plot we can see the extent of the inversion, with the dry bulb temperature going from 15°C at the surface to 18°C at about 2700ft, with a massive drop in dewpoint marking the top of the layer.