(You're welcome!) The 183 days officially made you a resident, but since you’re not a U.S. citizen, leaving the U.S. could mean you’re a non-resident going forward. As long as you’re residing permanently elsewhere (not intending to return except for visits), and not maintaining what anyone could construe as a tax home in the U.S. (the 401k and brokerage ownership alone wouldn’t suggest you’re still living here).
The IRS tells us about residency ending dates here, in Publication 519. Note that it normally takes place on December 31, but there’s an exception, wherein it can be your last day in the U.S. That exception requires having a “Closer Connection to a Foreign Country,” but if you were in a third country during the six month period between leaving the U.S. and establishing your residence in the U.K. (?), that may not apply. Also note that a statement is required to make the residency termination official.
Assuming you were a U.S. resident for all of 2022, you’d file a 1040 (like you have been), reporting all your worldwide income. (And you would approximately avoid double-taxation of the U.K. income by claiming the Foreign Tax Credit, described here.) But if you have been a nonresident as of 2023, you probably won’t have to file a U.S. return at all going forward (unless you come back and conduct business here), as the IRS discusses in this link.
I hope this clarification is helping, @fggrau94. (¡Viva Messi!)
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