The fog and the marine layer do filter out some UVB, but more importantly, its cold up there where you are for much of the year. Their skin needs to be warm for the UVB to work its magic. This is why we usually tell people to mount the indoor UV near the heat lamp. Putting a tortoise outside on a foggy or cold cloudy day is pretty pointless.
the portion of the process that is dependent on temperature occurs after the portion dependent on uvb. so outside exposure to uvb followed by indoor basking could possibly be a viable method when uvb isnt available inside. also, the temperature dependent portion can occur at temperatures substantially lower then what is frequently cited as 'basking temperatures', it just happens slower.