I remember the early e-commerce days on the Web and about this one UK website that had a glitch, which caused an unbelievably cheap bargain on their website, and many hawkers bought this cheap product and the website honoured it as they had to. Then a year later, Amazon.co.uk had a similar issue, which down to human error, caused a product listed as £299.99 to be listed at £29.99. Hundreds of people "bought" this product, Amazon did not "honour" this mistake price (apparently) as they had a clause in their T&Cs saying that they will only accept the contract of purchase when payment has been taken, and until payment has been taken the price may vary. So Amazon.co.uk got off scott-free!

The same would occur here, if BA are not accepting your payment, then don't have to honour that price.