I normally make all my purchases directly on the airline's website but on this occasion I'm about to book flights for my daughter and her friend and the prices (for the same flights) are quite a bit lower on a variety of travel agent sites.

Four of the five sectors are with BA, the other is with AA. If I book using a BA Chase VISA card will the booking still attract the 2.5 points per $ or does this only apply if booked on the airline's own web page?

The Chase site seems to suggest it's any BA purchase but it does not seem to stipulate whether the purchase must be transacted on the airline's own site.