Phuket is not only the dive capital of Thailand, but a major worldwide center for diving. Online directories list as many as 50 scuba centers, many with retail operations, concentrated on Patong Beach. You will be able to find all the major equipment brands.
For something simple like a weight belt, you can expect prices to be competitive with the UK— which is to say they will be more expensive than what you can find in the US or online. Inflation and the fluctuating strength of the baht make it difficult to say for certain. I will say that of all equipment, a weight belt would be one of the last items I would purchase. Many modern BCs will include some sort of integrated weight system of pouches, but there isn't much variation otherwise. I have never heard of any operation that will not lend you weight for a rather nominal price.
I have not been to Ko Tao in a number of years, but it has far fewer operations than you would find on the mainland, and so you can expect prices to be higher.