I was surprised / ambushed by this offer on a recent telephone booking too.

Luckily we knew roughly which hotel we wanted to stay at, but if you search Expedia in advance they can ask the agent to add any hotel available there on to a flight booking. Telephone booking fees waived and Atol protection added as the hotel addition makes it a package.

10x Avios per £1 spent on the hotel portion (including 10x points on hotel taxes and fees), then another 3x points if paying on the BA Amex.

The 3x points are credited in the usual monthly run, the 10x points apparently 35 post-departure. Fully expect to need to chase those up!

Not a bad surprise.