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.