Modern British Mystery

A friend of mine started a book back in the late 90s. It was about a character who was British & discovered his father was the president of the United States. The main character was based in London & an adult. An adult book - not YA. She thinks it was a newer publication date (at the time) mid to late 90's. Genre (she speculates) was mystery.

There's The Negotiator by Frederick Forsyth (1989), but the son isn't secret, he's just in England to attend Oxford.

There's Special Relationship by Robyn Sisman (1996), where the son didn't know who his father was, but it seems to be described more as romance or chicklit than as mystery.