You may find it more useful to move your post to the BA forum.
Most governments (UK, Australia, Canada) have already issued travel warnings against non essential travel to Bangkok and most of Thailand on May 13. So you knew the situation in Thailand when you booked. You would not have been able to purchase travel insurance for your trip also. BA may allow you to change but I doubt if they'd give you a full refund. It would not have been fair to BA's shareholders. If you've booked prior to issuance of the travel warnings, it may be different.