There is a tendency for some to overdo what they think of as 'the Oxford accent' and I suspect that was coming across. Being a tad pretentious sadly happens in many social contexts.(Original post by Pars12)
I'm interested how you know they were "properly upper class". Did you ask them whether they went to a public school?
A lot hinges around your definitions of terms like "well spoken".
I used to find that my accent changed between home and Oxford. I also found that my accent changed when I spent some time in Australia. Perhaps you were picking up on that. I think a lot of students 'sound' posh at Oxford. Doesn't make them upper class. Not even privileged in some cases.
Loughborough at number one