capital gain on US shares

Dual citizen here, UK resident and domiciled.

Many years ago I worked for a privately-held US company, and was partly paid in equity.   This company got acquired for cash recently, generating a six-figure capital gain on my shares.

Which country will get "first dibs" on the gain - in broad terms, will I end up paying 15% of the gain to the IRS and 5% (20% minus a foreign tax credit) to HMRC, or will I pay 20% to HMRC and nothing to the IRS after the foreign tax credit?  Or will something else happen?

I ask because if I'll be paying full whack to the IRS, I'm going to need to make an estimated US tax payment before the end of the year, correct?

If you are on the arising basis, the UK has the primary taxing right. You'll want to pay the UK CGT by 31 December. You might still owe some NIIT to the IRS so may also want to pay a US estimated payment.

Am I right that saying when calculating the capital gain one must determine the original cost in £ of the shares using the exchange rate on the day, and the proceeds of the sale in £ on the day of the sale?

This could result in a much higher £ capital gain than $ gain because of the fall of the £ against the $ in recent years.

Can excess FTC on cap gains be carried over to subsequent years?

If you are on the arising basis, the UK has the primary taxing right. You'll want to pay the UK CGT by 31 December. You might still owe some NIIT to the IRS so may also want to pay a US estimated payment.

Ah, the dreaded NIIT.   Looks like I'll need to make a payment to the IRS after all.  Thanks guya!