Minimising Uk Witholding Tax On Royalties

I arrived recently on an OA retirement visa from Australia. I am now not considered tax-resident in Australia (or the UK). I earn royalties in the UK on which withholding tax (22%) will now be deducted. If I was tax-resident in Thailand, this would be 5% (I think). What paperwork would a UK company paying royalties require from the Thai tax office in order to deduct withholding at the lower rate? Would involving the Thai tax office be advisable?