Many (most) UK prisoners in Bangkwang have refused to be returned to the UK.

The text of the UK Treaty is not the problem for the prisoners.
Their problems have more to do with the way the sentences are handled ... once the prisoner returns.

The Transfer Treaty was agreed by Thailand on January 22, 1990.
Implementation began on February 6, 1991.

Currently UK prisoners must wait for 4 years or until 1/3 of their sentence has relapsed before becoming eligible for a transfer.

If they have been given a "life" sentence by the courts (regardless of whether this has subsequently been reduced by a Royal amnesty) the minimum qualifying period is 8 years, though unconfirmed reports say that this only applies to drug-related cases -

Someone convicted of murder, who received a "life" sentence, would apparently be eligible to transfer back, after only four(4) years. UK prisoner relief groups continue to press the Thai authorities for a change in the transfer eligibility rules, but these could only be changed by an Act of the Thai Parliament.

If a prisoner, with a life sentence, decides to take advantage of the Prisoner Transfer Agreement, the UK is bound by the terms of the bilateral treaty to continue to enforce the Thai sentence.

Under the terms of the treaty, the receiving state can only continue to impose the maximum sentence issued in their own country for the same offence, rather than reduce the sentence to what would have been imposed had the individual been arrested in that country.

In cases where a prisoner, arrested on a drug trafficking offence, has been given a "life" sentence or a sentence of longer than 25 years, the UK is bound to honour the sentence issued by the Thai court, as the maximum "life" sentence in the UK stands at 99 years .
(although "life" is hardly ever given in practice in the UK).

Returning prisoners are treated in the same way as prisoners sentenced in the UK. They are eligible for release on parole after serving half their sentences.

Any review of sentence is a matter for the King of Thailand.
It is still possible for repatriated prisoners to appeal for clemency and to benefit from any amnesties. ( However, it has never happened ).