Change of Residency in an EEC country

[Virtualrecluse]

Background: I have been resident in Spain for three decades. Fiscal residency is part of it. My UK accountant recently asked me whether I had filled out a form at the time I left that confirmed to the UK authorities that I was no longer resident (I forget the exact name/number of the form). In any case, I did not, but that did not seem to present a problem as far as he was concerned. My question here is the following: I have read that, in the event I decide to renounce my Spanish residency (I am considering relocating to Thailand either to resume a teaching post that I had to leave last summer owing to family matters and Covid, or to go on a retirement visa (the 800K bank deposit type), the Spanish authorities automatically assume you have returned to your country of origin and notify the UK Govt.  Is that the case? Where can I get informed about it? What if you decide to reside in a third, specifically non-EU, country?