Time in US "on business"

Fortunately I do not have any US bank accounts or any US assets of any description. Previous years I am filing my time actually on business in the US is only a few days and my salary was relatively  low so I think I will just take the hit and apply to my standard deductions, personally.

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If your US source income is less than your standard deduction and exemptions you don't have to do anything. No US tax to pay. If you paid UK tax on the income you earned while in the US on business then you can resource and use FTCs....again no tax to pay.

A resourcing Form 1116 can only be used to avoid double taxation. If income has not been taxed by the UK it could not possibly be treaty resourced.

Interest income isn't really UK tax free, it's taxed in bands with the first band being 0%, no differently from the personal allowances that both countries allow for general income.

As such, I'm not convinced by your argument that income taxed at 0% cannot be resourced, but I'm open to persuasion. Can you provide a citation that supports your point of view?

Note that the treaty supersedes domestic law in both countries and explicitly provides for the ability to resource certain types of income, so any limitations on resourcing would therefore have to be written into the treaty itself.

Paragraph 6(d) of Article 24 of the treaty provides in part:

[F]or the exclusive purpose of relieving double taxation in the United States under sub-paragraph (c) of this paragraph, * * * income * * * shall be deemed to arise in the United Kingdom to the extent necessary to avoid double taxation of such * * * income * * * under sub-paragraph (c) of this paragraph.

It is the "to the extent necessary" language that limits the resourcing of the income to be just enough to avoid double taxation of that income, and not being able to resource more than that amount.

I would ring a tax accountant, many companies offer free advice and it is a safer option

Also, the re-sourced income goes in its own foreign tax credit basket so that it cannot be used to free up taxes in other baskets. See the re-sourcing box to be checked at the top of Form 1116.