His domicile, from the UK perspective whether of origin or of choice needs to be established before going any further. Any hint of a change of domicile or of a domicile of choice lapsing during his lifetime needs to be carefully checked as to its consequences.
If he is, and remains, domiciled outside one or all of the United Kingdom jurisdictions by origin or by choice, then you might be able to rely upon each will being operative alone, subject to any residency or other legal link in India, Dubai or Thailand. Note that putting the succession in Dubai will need careful thinking through with local advice.
However, is the UK will a catch all device, or limited only to UK situs assets? If a catch all, that could be indicative of his wishing his overall position being subject to one of the laws of the United Kingdom, thereby evidencing a domicile.
If the “UK” Will deals with UK situs assets, I do not see how the executor can avoid submitting it to probate in the UK and at that point giving a statement of domicile at death. If there is an offshore holding of UK realty, which is not longer excluded property, then whilst there will be technically no probate in the UK required over the foreign interest there will be an IHT return to file on his decease.
If it deals with a residential property in the UK you might now find any claim of a foreign domicile queried, as a substantial link with the situs jurisdiction in the United Kingdom will thereby be evidenced, even through a foreign holding company, unless you can show that the domicile of origin if any was in another UK jurisdiction - the Scot/Anglo-welsh divide - and then face queries being followed up in the absence of any UK resident income tax returns,