Perhaps this has always been the case and I am just a bit slow, but the leaflet with my Tesco Visa statement this month says that holders of a Platinum Tesco Visa get 2 points for every £2 spent on tesco.com.

This works out at 2.4 BA miles per £1 of spend - far better than the BA Amex! It is not clear if this works for everything you buy though the tesco.com portal, such as flights from lastminute.com.