You asked: I have a question about state taxes. I worked 1/3 of the year in the U.S. and then moved to the UK temporarily. I've decided to keep my US residency as the move is quite temporary. But I do work for a UK firm and pay UK taxes and get NHS and all that.
I've done my federal taxes (on Turbo Tax) and I've been able to count the taxes paid to UK there. But on my state return, Turbo Tax isn't showing any kind of credit for those, and thus I have a big state tax bill due. Does this sound right? or is this Turbo tax not being particularly wise?
It is probably a mistake to claim UK tax as a credit (form 1116) rather than exclude your income (form 2555). Ask for an extension until you've been abroad long enough to qualify for the exclusion, which will then reduce your AGI (adjusted gross income) for state purposes as well.
It's true that many states tax their expatriates abroad (NY, CA, IL and others do not, if they've been abroad around 11 months or more). But unless you are doing business from a permanent establishment in a state, why volunteer to pay tax?
NI contributions are dealt with in the US-UK totalisation agreement. Unless you work for a multi-national firm that has contracted with the SSA to pay FICA to its workers abroad (this is because of defined-benefit pension arrangements integrated with Social Security; most companies have moved to 401(k)s now) you do not have to pay both NIC and SET. It is rarely in your interest to do so unless you are trying to finesse the system or need quarters of coverage.
NIC and VAT would only be deductible for US tax purposes if they were business expenses.
TurboTax does not deal well with SET issues; there is a workaround by clicking the box to say your Sched C income came from "notarial services" and then with pen-and-ink changing your return to refer to the Totalisation Agreement. But I don't think that's you problem.
So: File a part-year tax return with your state ending on your date of departure. Unless you come from one of those greedy states that want to tax their expats forever...