Share identification rules for foreign tax credit relief

I have two units of stock of the same company in the S104 pool. Both of them were acquired for £100 but one of them was charged foreign tax of £20 on acquisition. I sold one of the units for £200, generating £100 of capital gains (I have other capital gains and income that brings me above the 20% capital gains threshold). Capital gains tax to pay is £20. For the purposes of calculating the foreign tax credit relief, should I: a) apportion the foreign tax based on the total number of shares that were subject to foreign tax, leading to £10 FTCR and £10 remaining foreign tax to claim once I sell the other unit, or b) consider the full foreign tax paid already, leading to £20 FTCR already ?

Hi,

As it is within S104, you will apportion the Foreign Tax Credit Relief to only claim £10 at present.

Thank you.