Hello Allan, I am Keith, one of the experts on Just Answer, and pleased to be able to help you with your question. I have a Thai wife and spend quite a bit of my time in the Kingdom, in fact i am just back a week ago.
Your domicile is British. It takes at least 16 years to change a domicile and then it is not that easy.
Firstly, when you left the UK did you complete a Form P85 and send it to the Inland Revenue as it was then? If you did not you should do so immediately, but of course these days to HMRC. Fortunately there is no time limit as to its submission, it is available on the web and can be filed on line. On receipt that Department will classify you as non resident and you will find dealing with them much easier.
You are liable for UK taxation on your net rental income and as a citizen of an EEA country you are entitled to the usual Personal Allowance, currently 11K, to offset this. For your tax return you must complete a SA100 and a SA105 for the declaration. Remember when filling in these forms you only need complete those boxes relevant to your income, it makes a daunting form much less complex.
Your UK assets will be liable to UK Inheritance tax (IHT) on death and anything over 325K be taxed at a flat rate of 40%. From 2017 to 2020 however for a house willed to your children the tax free element progressively increases to 1 million.
I do hope that you have found my reply of assistance.