Finished Secondary School in 87. Here in England we finish school at 18. 87 - 18 = 69.
Moved to London in 87. He started University in London that same year.
So Mike was born on or after September 1st 1968 and before September 1st 1969, making him 46-years-old (except in the unlikely event that he was born between the 1st and 3rd of September, in which case he would be 47).
Mike's fee-paying boarding school in the heart of rural Kent: