Its not often I back up Tourist but...in this case.
The weather conditions in SW Cornwall are subject to many influences due to its position in the Gulf Stream and its general elevation.
In summer months, advection fog can indeed be an issue dependant on the direction of the wind and banks of fog can drift up from the coast over parts of the airfield whilst other areas are clear.
In winter months, Fog is often due to low cloud (warm front) combined with the elevation of the aerodrome, leading to the famed 'gale force fog' phenomena. When this is the case, as Tourist has already mentioned, approaches can be made VFR from Loe Bar or up the Helford River.
Quite often, when a precision approach cannot be completed, Culdrose ATC are able to descend RN helicopters over the sea until they are VFR, quite legally iaw JSP552. the criteria if I recall corerectly are for RN aircraft or aircraft under contract to JSATO, with a units AAIA or Dnager areas, 2 miles clear of the coast for RW or 5 miles for FW, down to 500'amsl. This often allows aircraft to get below a warm front that is causing low visibility over the airfield and navigate VFR to the airfield. The SAR dispersal is slightly lower than the rest of the airfield and it is very much a hop over the fence from the Helford river.
The problem will occur when Civil SAR takes over and the legal application of the JSP 552 rule, which will probably not apply, thereby removing the option of IMC descents over the sea. there is parallel work ongoing however in order to authorise the use of GPS descents to a known point, to enable approaches to aerodromes without ATC services.
Rotary wing operations to Culdrose are rarely interupted. Weather can be a problem for FW, with the higher DH and no VFR option but RW aircraft have many options, including the use of different coast approaches dependant on the prevailing wind. Culdrose is an ideal SAR base as has been demonstrated over many DECADES of use of the aerodrome. Its just not much kop for airshows!
Spelling mistakes are because I cannot be bothered!