Cost basis for stock acquired prior to UK residency

I acquired stock in a single company prior to moving to the UK. Now, as a UK resident, I sold some of that stock. What is the cost basis I should use when calculating the sharepool value? A concrete example: I purchased 5 stock units in Company A for $10 each prior to moving to the UK. 1) I read somewhere that the cost basis is set to 0 for stocks acquired while not a UK tax resident. Is that correct, or should I use the purchase price ($10)? 2) If I use the purchase price, what exchange rate should I use - that of the date of acquisition or of sale? Thank you!

Hi,
It is the original purchase price and the exchange rate is at date of purchase.
Thank you.

If the stocks were acquired as part of an employee equity compensation program that grants RSUs, where stocks are granted and then vest at a later date, does this change the answer? If not, is the purchase price (and therefore cost basis) that of the grant or vest date? Thank you!

Hi,
As the payment is from your employer, the income should be shown in the employment section if it is included in your P60. You would then claim credit for the tax in the foreign section under 'Employment, self-employment and other income which you paid foreign tax on'.
If it is not included in your P60, please include it in the box on the employment page for 'Tips and other payments not included on your P60'. The guidance below advises that when RSUs payout at the market value on what is called "dividend equivalents" in either cash or shares, such payments will generally be taxed as earnings in the year they are received.  
ERSM20193 - Employment-related securities and options: what are securities: RSUs and dividend equivalents
Thank you.