You are being quoted a price on the basis of starting and ending your trip in DUB. Finishing in LHR would cost more, so I don't think BA would be over the moon about your plan... I haven't heard of anyone being taken to court to pay the difference but I have heard of BA suspending the right of a travel agent to issue tickets with them and a debt notice being issued for the difference. They also told the most famous BAEC blogger that they want to/are planning to crack down on it. Cost to BA of suspending your BAEC account?
I understand AA have a specific condition that dropped last sectors can lead to all frequent flyer miles for the whole trip being forfeited. If you choose to drop that last flight you take your chances, however unlikely it is that you will pay a price.
volenti non fit injuria.