When a tube station or line is closed for planned work (or there's
other unplanned disruption), and TfL advise that tube tickets will be
accepted on local buses, how does the Oyster PAYG system cope?

Wood Green tube's closed this weekend, so instead of walking to the
station, I'm taking a bus to Turnpike Lane. While the cost of two PAYG
singles already take me beyond the z1-3 cap (with a 16-25 railcard),
I'm curious to know what would happen to the bus fare I didn't reach
the cap.