I recently joined BUPA and had a good discussion with the broker.It is a worldwide policy (except USA - go figure). If you require treatment etc in any place away from their list of registered hospitals you must pay up front.You then notify BUPA.
You must get receipts for everything and a covering letter from the Doctor who was in charge.You then submit a claim to BUPA and they pay you the money back.
When you get the policy it comes with a booklet with all the participating hospitals listed in it by regions of Thailand.It is also very clear as to what is covered and what is not