Thai Wife's Widows Pension

Hi! Any genuine information on my query very much appreciated. Hopefully there are Brits out there in a similar situation. We both reside in Thailand.

I am from the UK and get a private annuity pension paid into a UK bank in £. When I die, my Thai wife, who has never lived in the UK, and does not have a UK tax reference, will receive 67% of this pension. It has to go through the UK tax system, and 20% will be deducted at source. Being a Thai passport holder, can she receive the UK tax free personal allowance? And if so, how would she go about it? There is a double taxation treaty between the UK and Thailand but I have a suspicion that this type of annuity pension is not covered for widowed Thai citizens.