Need a bit more info Nan. W/R/T the hypothetical tax owed to the US, is this income you received BEFORE the move? And have you worked in the UK SINCE the move.
Generally, we pay the UK first, then take a credit to offset any US tax that might be due. Income you earned in the US BEFORE you became UK resident is not UK taxable or reportable. So, how are you doing these theoretical calculations? (Make sure you aren't reporting income to the UK that isn't reportable.)
Another point - for 2017, if you are SURE you will owe US tax, it needs to be PAID by April 15th, but you don't actually need to file a tax return until a few months later with the automatic extension. If you ten find you overpaid, you will get a refund just as you would have in the US.
Again, need a bit more info to (hopefully) point you in the right direction.
I only counted income for months I was in the UK. I arrived on April 30 this year, so I counted from May 1 forward. Not sure how they are going to count it, but I was on paid vacation for all of May, off the payroll in June, and officially retired as of July 1. In my calculations I assumed that the May vacation pay was going to be considered taxable income by HMRC.
I had no income for June or July. Beginning 1 August of this year I now receive pension payments. I also took about $11,000 out of a 401K to pay off USA credit cards I ran up in June and July, plus the earlier cost of the actual household move. I have not done any paid work in the UK.
I will be taxed in the US, that's a given; however, the payers withheld approximately $7,200 from my Jan-May salary, my pension, and the 401K disbursement (at 20% of the disbursement). So I believe I'll be getting a refund from the IRS (and my ex-state). If all goes well, the refunds will be enough to cover the difference in the HMRC and IRS taxes due to HMRC for that period of time.
Since I won't have paid HMRC anything before the end of 2017, I can't claim a credit against 2017 USA taxes for the relevant months of 2017, right? I was thinking that for this first filing I'd take a credit against HMRC for USA taxes paid, but in subsequent years be taking the credit for HMRC paid against USA taxes owed. Otherwise, I'll end up paying taxes to the USA for calendar year 2017, and then overlapping taxes for the May-Dec of 2017 to HMRC.
It's all in how to handle this first year.