I registered for residency here in Portugal. I've got the piece of paper to prove it.

Does this automatically inform the UK I am no longer resident there, or do I have to do something to inform them?

i.e. how do the UK customs, the NHS and UK insurance companies know I am no longer there? Is there a central database they can check?

Short answer is no they don't know unless you tell them. The paper you have is purely Portuguese internal. You must take the initiative and tell the UK . After 183 days you are tax resident as well so get hold of a Double Taxation Form from the Inland Revenue and fill it out and send it back to avoid being taxed there and here.

Congratulations! Can you give us some details about the process, what documents you needed, and income you had to prove etc.?

In that case and you tell no one n the UK, and if your Portuguese Fiscal address is in the UK at time of taking out residency, how does the Portuguese IR find out?

Remember: tax evasion will land you in trouble.

yes of course you are right, but if you do not live in Portugal for more than 183 days then you are not a Tax resident??

Yes, of course but you will be tax resident somewhere? Even if you don't spend more than 183 days anywhere, it's not a loophole.

I am not trying to evade paying tax, its a question of where I would pay any tax.
For me, Portugal tax rates are much higher than the UK. That is all I am trying to get over.
If I was to rent out my property in Portugal then of course I would have to pay the tax on the income in Portugal, and UK. But assume double taxation would mean that no tax would then be due in the UK.

You pay income tax wherever you are resident. If you’ve moved to Portugal and are a resident there for tax purposes then that’s where you pay tax. Complete Form P85 to inform HMRC you have left the UK. It’s optional but you’ll usually receive a tax rebate.

But as I understand it you can be a resident of Portugal, but if you spend less than 183 days in Portugal and more than 183 days in the UK, in a year, with a home in both, then you are a tax resident of the UK.
I stand to be corrected but that's my understanding

Not necessarily. If you have a home in Portugal on 31 December (one you use to live in rather than rent out) then I wouldd expect you to be treated as Portugal resident, irrespective of days in the country. And as far as the 183 days is concerned, remember that this refers to a rolling 12 month period and within that period days spent here are aggregated - they don’t have to be continuous.

Each midnight in the country is a ‘day’ of residence so, for example, if you left at 11pm ona Tuesday, that Tuesday would not count. If you arrived at 11pm and left at, say, 1am on Wednesday, then the Tuesday would count as 1 day here.

Equally, I have a property in the UK, which is where I am resident, with GP etc etc, and that is where I spend more time in any 12 month period. % months Portugal 7 mths UK.
But as we know one is supposed to declares ones presence in Portugal after being here for 90 days, hence becoming a resident.
For me this is health reasons, winters in Uk are too cold and summers in Portugal are too hot, so all I am trying to do is to be able to do what I have done for years, without becoming a burden on the Portuguese health service. In case of residency in Portugal I would take out proivate health insurance as a resident of Portugal. And whilst in the UK continue to use the NHS in the UK as at present.

I seem to recall reading that an absence of more than 6 months can nullify residence, though that may apply more when a visa/permit is required. Then again, Brits are to become third country nationals so who knows?

Well that may be another issue at a later date, by which time I may have decied finally where to remain. But in the meantime I will not take out residency for the moment, and see how things go during the transition period.

Tax resident in the UK, yes.