i got in for ppe this year but i didn't have a mentor or anything like that - i know that ep uses the tsa as the entrance test too which is just not very difficult to prepare for since the difficulty is just timing and exam technique (so do the past papers and practice) whilst the actual qs themselves are not that hard
i would also imagine that ep is not asking anything massively technical at interview? you could try signing up to zero gravity to see if you can get a mentor who does ep/psych at oxford and also sign up to the oxbridge launchpad if you really feel that you need the extra support