Heard from HMRC on the pre-tax deductions taken from the USA paycheck for vacation pay disbursed to me while I was in my first month in the UK: if it was pre-tax in the USA, they consider it pre-tax (or excluded from tax) in the UK. So, I go with my USA taxable income as "foreign income" for HMRC for that month, and yes, they will allow a tax credit for tax paid for that month only on my 2017/18 taxes.
Now, another question: I do not yet draw US Social Security (I'm holding off so that I receive higher checks each month in a year or two). When I do draw it, I know the UK will have first dibbs on taxing it, and it won't be taxable in the USA at all because of the treaty. However, if it ~was~ taxable in the USA (if I was living there), about 25% of the social security payment would not be taxed. I remember reading something about if a benefit payment is non-taxed in the country of origin it should be treated the same in the country of residence (but it's been a while since I read that and can't find it again). Anyone know anything about that?