Brits in the US

Usually under the substantial presence test — a day count across the current and two prior years — or from the date you take a green card. Both can land mid-year, which is why the arrival year needs planning rather than reconstruction.

If you keep UK-source income such as rent, or remain UK resident under the Statutory Residence Test for part of the year, yes. Leaving does not close Self Assessment by itself.

The substantial presence test counts all days this year, a third of last year's and a sixth of the year before. Reach 183 on that weighted count, with at least 31 days this year, and you are generally resident.

Not usually. Most visa categories are irrelevant to the day count. Students and certain exempt individuals are treated differently, which is a genuine exception rather than the rule.

The year you arrive or leave, where you are non-resident for part and resident for the rest. The two parts are taxed on different bases, and the split point is worth planning rather than accepting.

Not to the IRS. The US does not recognise the ISA wrapper, and the funds inside are usually PFICs with their own reporting. Many people close or restructure them before moving.

Yes, and the Statutory Residence Test decides whether you actually have for tax purposes. Leaving physically and leaving for tax are not the same thing.

California is the usual difficulty — it does not follow federal treaty relief and applies its own residency test. New York is similarly demanding. Several states have no income tax at all.