I'm with Mach on the fog and Wick
I'd recommend going to Wick as Andrew Bruce and his team have a fantastic wealth of knowledge. His company - Far North - is more of a US style FBO than I've seen anywhere else in the UK. We recently got stuck in fog for three days whilst trying to get to Shetland and Andrew arranged hotels, taxis etc.
I certainly would recommend immersion suits as well as dinghies. The water is really cold up there even in the summer. I used to sail small boats a lot and I always found that a number of thin layers under an immersion suit worked better than bulky woolly bears or the like if you find yourself in the oggin. Technical clothing has improved a lot in the last decade or so.
On the fog topic, be very cautious before you set out. When it rolled in on us unexpectedly it not only took out Wick, but all of Orkney and Shetland and even Inverness. You might find your alternates seriously restricted. As other posters have said - ASK before departing.
Notwithstanding the challenges, it is beautiful flying in that area and well worth the effort. We shall be up there several times this year.