As an employee, your former employer is obliged to send you a P45, (3 parts). This can be emailed to you, (password protected) soon after your final payment. You should be able to do a rough estimate of your tax liability from worldwide income to the date your left the UK, (providing you meet HMRC's SRT as a non resident). Your UK earnings will have no impact on the IRS return for 2024, as you only became a 'U.S. Person' on arrival, (unless you voluntarily opted to file MFJ with your U.S Spouse). As such however, you now declare your worldwide income to Uncle Sam, to include FinCEN114 if you hold more than $10,000 outside the U.S, even for a day (simple form).