re-sourcing income from USA to UK - accrue v cash

Because of the UKs quaint tax year end on 5 April; electing the accrued method for FTC is generally an absolute disaster. Don't go there.

Ok, do NOT check accrued on the 1116. 

The question remains -  Can I file the 8833, 1116, and payment receipt with my USA 2017 tax return and get  tax credit for the period of  May-Dec 2017 from amount I pay HMRC in, say, April of 2018?

The publication to which I am constantly referred says that foreign tax usually accrues at the end of the foreign tax year. Which seems to indicate I cannot do the above. But there is the "usually" in there, and IRS and TurboTax saying that I CAN file after the end of the UK tax year (which is well after the end of the US tax year).

Ok, do NOT check accrued on the 1116. 

The question remains -  Can I file the 8833, 1116, and payment receipt with my USA 2017 tax return and get  tax credit for the period of  May-Dec 2017 from amount I pay HMRC in, say, April of 2018?

The publication to which I am constantly referred says that foreign tax usually accrues at the end of the foreign tax year. Which seems to indicate I cannot do the above. But there is the "usually" in there, and IRS and TurboTax saying that I CAN file after the end of the UK tax year (which is well after the end of the US tax year).

No. The UK tax for 2017-18 is the amount accrued for 2018. Have you read the Treasury Regulations of foreign tax credits possibly? They are incredibly long; but incredibly well written and therefore not too tough to understand. TurboTax has no legal authority.

I have read all the IRS pamphlets.

In redoing my UK tax to remove the employment/vacation pay, it appears I will owe HMRC about 2,200 pounds for 2017/18. That's (at $1.40) about $3,080.

I paid the IRS about $5,900 dollars in tax for Jan-Dec 2017. Of that, roughly half is attributable to retirement income. (About $2,900).

So, if my US tax liability for 2018 is going to be around $2,300, I get to subtract that from $3080. Which gives me an excess credit of about $780 to carry back to 2017 in 2019.

$2,900 - $780 means I'm being double-taxed to the tune of $2,120.

SO, there is supposed to be no double-taxation. There is a treaty in place, is there not?

I have read all the IRS pamphlets.

In redoing my UK tax to remove the employment/vacation pay, it appears I will owe HMRC about 2,200 pounds for 2017/18. That's (at $1.40) about $3,080.

I paid the IRS about $5,900 dollars in tax for Jan-Dec 2017. Of that, roughly half is attributable to retirement income. (About $2,900).

So, if my US tax liability for 2018 is going to be around $2,300, I get to subtract that from $3080. Which gives me an excess credit of about $780 to carry back to 2017 in 2019.

$2,900 - $780 means I'm being double-taxed to the tune of $2,120.

SO, there is supposed to be no double-taxation. There is a treaty in place, is there not?

We are not discussing the treaty. The saving clause precludes that being relevant in any fashion in this discussion.  IRS publications have no legal standing. Treasury Regulations - conversely - do carry the benefit of substantial authority.

Treasury regulations???