The UK/Thai DTA is quite clear to me, having just found it. for the purpose of Income Tax it was effective from 1981 and Article 4 (fiscal domicile) states:
Article 4
Fiscal Domicile
(1) For the purposes of this Convention, the term "resident of a Contracting State"
means, subject to the provisions of paragraphs (2) and (3) of this Article, any person
who, under the law of that State, is liable to taxation therein by reason of his domicile,
residence, place of management, place of incorporation, or any other criterion of a
similar nature.
(2) Where by reason of the provision of paragraph (1) of this Article an individual is a
resident of both Contracting States, then his status shall be determined in accordance
with the following rules:
(a) he shall be deemed to be a resident of the Contracting State in which he has
a permanent home available to him. If he has a permanent home available to
him in both Contracting States, he shall be deemed to be a resident of the
Contracting State with which his personal and economic relations are closer
(centre of vital interests);
(b) if the Contracting State in which he has his centre of vital interests cannot be
determined, or if he has not a permanent home available to him in either
Contracting State, he shall be deemed to be a resident of the Contracting State
in which he has an habitual abode;
(c) if he has an habitual abode in both Contracting States or in neither of them,
he shall be deemed to be a resident of the Contracting State of which he is a
national;
(d) if he is a national of both Contracting States or of neither of them, the
competent authorities of the Contracting States shall endeavour to settle the
question by mutual agreement.
(3) Where by reason of the provisions of paragraph (1) of this Article a person other
than an individual is a resident of both Contracting States, then the competent
authorities of the Contracting States shall endeavour to settle the question by mutual
agreement.
I have a permanent home available in the UK whereas I do not have such in Thailand where I rent a condo as a tenant on a temporary yearly basis plus my connection with the UK is stronger because I have immediate family connections there. So in my book, I am a UK Tax Resident.