1. His old seat of Henley is very much taken.
2. The current MP for Uxbridge is standing down, so the seat is vacant.
3. Uxbridge and South Ruislip is in west London.
4. Uxbridge is a pretty safe Conservative seat.
5. Uxbridge and South Ruislip is in outer London.
6. The bookies think he's going to stand in Uxbridge and South Ruislip.
OK, it's not exactly hard evidence, but definitely worth pointing out: The most likely odds, according to Ladbrokes, were that Johnson wouldn't stand.
Now that option is probably not going to happen, the bookmaker reckons he's most likely to stand in Uxbridge.