By the amount of replies here, I hope you found your answer. But yeah, relationship level only seems to increase through actual events, for the cornwall thing I just made sure she got each trait before doing it.
Well I got her Submission to 10 then. TBH I was not pay too much attention, it was something with a 0, at which point I figured 'good enough' or hoped.... and it seems it was.
But good to know. I need Sluttiness and Exhibitionist skills of all the girls to be high enough to unlock certain things still locked (like the Twins Doggy Girl/Cat Girl photo session for example)... and was wondering just how high I needed their Exhibitionist stats... so 10 is the magic number eh? Woot.... weebs are gonna love this next photo set of them :P