@spa2000
The DTA is here:
https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/507409/spain-dtc_-_in_force.pdf
I think you're slightly misreading the DTA. How you become a tax resident of Country A, or Country B, is, generally, outside the scope of a DTA between Countries A and B. You ARE a UK tax resident because you still live there, and you never stopped living there, so you've never lost this status. But you ALSO ARE a Spanish tax resident by virtue of spending most (183 + days per year) of your time there (i.e. the "substantial presence test"). However, the DTA does say that when you're a tax resident of both, there is a "tie breaker", as you mention, to establish your primary tax residence country. You seem to be taking it to mean that you are therefore NOT a Spanish resident for tax purposes, and don't need to pay Spanish income tax, regardless of your time there. That might sound pedantic of me, but it might explain why there is confusion, and the need for multiple accountants.
The DTA seems pretty straightforward to me, and it should not require a team of accountants to go through it and implement your tax payments. Apart from anything else it's really expensive to pay 3 or 4 accountants / tax attorneys. And "everyone has an opinion" so they're almost bound to disagree. :-) Better to have just one... but a good one.
I stand by my previous advice that you're not likely to be an "autonomo" if you have a Spanish company, even if this might have "lower social security payments". If you have a Spanish (or UK) company, the usual options are to pay yourself some combination of (a) director's fees (as a director), and/or (b) salary (as an employee), and/or (c) dividends (as a shareholder). Instead, treating yourself as a freelancer/contractor of your own company seems rather cumbersome/unorthodox.
Or, if you think the tax situation is especially complicated, you can even choose to pay yourself nothing (from Spanish company), as you pay yourself enough (or more) from the UK company. But, in this case, it would be recommended to have an appropriate payment - "management charges" or similar - from the Spanish company to the UK company, to reflect that you spend most of your time in Spain.
Even if you need TWO companies (I can see this, now that UK is outside of the EU), it doesn't mean they should be two independent companies (with a common shareholder, you). One could be a subsidiary (e.g. UKco owns Spainco) of the other. Even if you keep two separate companies, you don't necessarily need to be an employee of both. Dividends are a well-established alternative for getting money out, whether or not you are an employee.
Either way, income from employment, and dividends, are both covered by the DTA.
Article 7 BUSINESS PROFITS - looks pretty easy. Your companies make profits (big ones, hopefully) and pay applicable corporate income tax in the country of incorporation.
EXCEPT that there is also Article 9 ASSOCIATED ENTERPRISES - which applies as you are the common owner of both companies.
Article 10 DIVIDENDS - tells you what to do if you take money out via dividends (my suggestion).
Article 14 INCOME FROM EMPLOYMENT - applies to you, as you're paying yourself a salary from both companies. This again covers the issue of 183 days (irrespective of your primary tax residence, per DTA discussion above) AND raises the issue of WHERE you work when you receive a salary! So, it seems it is as I previously suggested, that you should be paying your Spanish salary net of income tax (and other deductions). But it seems this also raises an issue that you earn a salary from your UK company, but are working mostly in Spain... so you need to carefully read the DTA to figure out who gets what.
There is also Article 15 DIRECTORS' FEES which is another way that allows you to pay yourself something,
In other words... "the answer" is in the DTA. No accountants required. :-)