Thank you to the OP for pointing out these fares: have just booked my Mum to visit us in BOS for 259UKP, when the same fare on ba.com was c. 200UKP more expensive. Of note, I think these are mistake fares, since they are only this price on expedia.co.uk, on expedia.com, they price the same as the BA website. The flights I bought booked into "O" class, but the cheapest fares on ba.com are booking into Q class.

On a related note, the US ba.com website has an "internet price promise": I wonder whether one could claim back on it, even if flights were originating in UK?

tb