Doesn't sound right to me.
£140 is the at-airport charge for a second (or subsequent) additional bag.
The first additional bag bought online should be £55 if the underlying allowance is one bag, or £120 if the underlying allowance is two bags.
http://www.britishairways.com/en-gb/...weight-baggage
As always, the big question is what does the e-ticket say?