Hi. No assessment is made yet. I've had advice both ways - yes it is, no it's not, taxable. From the HMRC! I'm going to go ahead and report it on the form, in the white space, and claim it as exempt from taxation as it was "for" the 2016/17 HMRC tax year when I was not resident, and, thus, is not taxable. (Their advice position: that it is leave pay and that I was resident when I received it, and so it's taxable. And: no, it's not taxable.)
I had no duties while in the UK. I was on paid vacation for a month, and the vacation began before I moved over. I know part of this particular sum is from an excellence reward for performance in the prior year, and that part for sure is not taxable (it is for 2016/17 UK tax year). If the rest turns out to be non-taxable, that takes care of my other issue, the $500 that the IRS has grabbed on my 403b income (that I'd rather went to HMRC than pay twice), because my UK income would be below the personal exemption limit of 11500 and so no taxes would be due this year (17/18) to HMRC.
As far as the split-year treatment, I haven't gotten that far. I was going to download the paper forms today and hit them, as the Daughter is away for the next two days and I'll have no interruptions. I assume I can elect split year by checking a box on the form? (I believe I was told it would be applied automatically and to not even report anything from before we became tax residents.)
I do not want to choose remittance, as I spent quite a bit of savings setting up over here, and moving US savings to UK savings (to hedge against the exchange rate going up) and would be worse off if I used remittance. So it's good to get that confirmed - I'm reading the manuals and translating correctly.
(I get a little twitchy about UK government manuals, because when I applied for a government job the wording in their paperwork was that they didn't "usually" hire people who hadn't been here 3 years, and when they offered and then rescinded the offer they wrote and said they had an iron-clad policy that they did not hire anyone under any circumstances who had not been here 3 years. I am a very literal person, so interpretations of regs that are not literal confuse the hell out of me.)