Exchange Rates SA100 & 1116

I will have 401(k) income this year and understand that I can use form 1116 (resourced by treaty) for any HMRC tax paid. I think that it’s mandatory to used the HMRC exchange rate to convert USD to GBP for SA? So, if that is the case, would that HMRC rate be used as the exchange rate on the 1116 also, or just use the IRS/Treasury rates as usual in converting GPB into USD, (meaning you’d use two sources of exchange rate – one from USD to GBP - HMRC and GBP to USD - IRS). 
I’d like to use the IRS/Treasury rates rather than those of HMRC, meaning one rate source for the IRS and HMRC. Does anyone do this, perhaps making a note in the comments section of the SA?
This would also affect any other baskets too, where tax is paid to HMRC on U.S. income.

I don't think it is mandatory to use the IRS published rate and in fact the average rate the IRS publishes each year should really only be used for periodic monthly distributions.  I seem to recall the IRS website stating that any reputable exchange rate source is acceptable, such as XE.com.  For my UK pensions I used to maintain a spreadsheet and convert £  to $ each date my pension hit, and I also calculated the amount using the published IRS average rate and I would choose which one suited me best when filing. I never specified which rate I'd used although the place to do so is on the substitute 1099-R used for the UK pensions.

https://www.greenbacktaxservices.com/blog/irs-foreign-exchange-rates/

The IRS has no official exchange rates. However, according to the IRS website, they generally accept “any posted exchange rate that is used consistently.” And when valuing a foreign currency that has multiple posted exchange rates, the IRS advises that you use whichever rate applies to your specific circumstances.

For example, if you’re reporting a single transaction from one day, such as the sale of a business, use the recorded exchange rate for that date. If you’re reporting income that you received evenly throughout the year, on the other hand, you’ll probably want to use the yearly average currency exchange rate for that tax year.

As durhamlad said, the IRS isn't picky about which rate you use, as long as it's reputable.

I've just used the HMRC rate to go both ways, so it's the same rate and internally consistent. I figure HMRC is at least as reputable as xe.com or whoever. I can't guarantee that approach is acceptable, but I've never been challenged. In practice, the differences are going to be tiny, so just using one rates feels right to cancel out even those tiny differences. I expect it would only become an issue if you were cherry-picking rates to save money on taxes - using xe.com for one transaction, Treasury rate for another, HMRC for another, etc. - pick one and use it consistently and I expect that avoids any issues.

FBAR does have it's own specific Treasury rate to use, that's a different case.

My question was more if it’s mandatory to use HMRC rates?

My question was more if it’s mandatory to use HMRC rates?

I think we answered your question by virtue of the fact that HMRC issues and maintains rates every month while the IRS only issues 1 rate  in January each year which is an average for the previous calendar year. I doubt very much HMRC are going to come after you no matter what rate you choose because $ figures never appear on the return, just £s after you have converted them. As long as you can defend your position if you should ever be audited then choose whatever rates you want, but I would personally stick to the HMRC rates even though it does not specifically say individuals must use the rates.

https://www.gov.uk/government/publications/exchange-rates-for-customs-and-vat-yearly

Traders, business and trade organisations should use these exchanges rates to convert any foreign currency to sterling for customs and VAT purposes.

You can find out what the recent and previous foreign rates of exchange are.

You can either:

view the rates online
download the CSV files

Thanks...
Looking at the average rate from HMRC - 0.7259, (I use the average) and the IRS 0.7270 it really is a very small difference.

Thanks...
Looking at the average rate from HMRC - 0.7259, (I use the average) and the IRS 0.7270 it really is a very small difference.

Yes, not worth bothering about.

Of course, if you really want to see if you can save a few pounds, then keep a spreadsheet showing your 401k distribution each month alongside HMRC's published average rate for the month.  If the total £s using the average monthly rate works in your favour use that method otherwise use the HMRC annual average. You can chop and change year by year knowing that whichever method you choose you can say that you used the official HMRC rate