My understanding is that tax paid in a foreign country, if the foreign country has the right to tax the income, is not a deduction from the income remitted into Thailand, but instead a credit towards any taxes payable.
In other words, the taxes paid in the UK would be a tax credit, not a deduction from income.
In this case, it seems the point is moot, but going forward it may be something to look at closely. I don't think current Thai income tax forms address this, however.