I thought I'd mention why I think it's unlikely GM would have hidden herself in the UK.
Simply, IMO, everyone who was anyone knew her. She went to Marlborough College (a boys "public" boarding school, what in the US we would call a private school). I actually have family who went there. GM could only have gone there for 6th form (grades 12 and 13), since it only admitted girls starting in 1968 for 6th form, and became only fully coed in the late 1980's. I haven't seen where she went before that.
Then she went to Oxford. You had to be "very clever" or well-connected and intelligent. GM was probably all of the above. You didn't have to have much money to go to Oxford in those days.
Those institutions are sometimes regarded as snooty now, but were not especially back then. Just not easy to get into, especially for women (there weren't many places for women at Oxbridge compared to men)
To give you an idea on lack of snoot, when I went to a well-known boarding school in England during the 1960's, it cost 200 pounds for everything, room, board, tuition, EVERYTHING for an entire academic year of 10 months. These schools definitely cost a fraction of a Swiss boarding school (which might have been a more likely choice) or an American boarding school. British boarding schools in that era were not a life of luxury, though. Ha ha ha trust me on that (picture 12 girls to a room)! and they were quite difficult to get into (consider my entrance exam at age 11: French, Latin, Algebra 2, and analysis of a Shakespeare sonnet on the English paper).
Hope that helps for understanding GM's schooling. The thing is with these public school and Oxbridge (Oxford/Cambridge)? People know each other. It's a connected world. They move in certain circles. Very difficult to be incognito.
So, the odds of GM keeping her whereabouts secret was very low.
To top it off, there's the matter of the bloodhound British press in general, and their constant hounding of royals, as in Prince Andrew...
IMO
