Crypto Cost

Hello, I deposited £50 Gbp into an crypto exchange and then purchased USDC which was worth less @ £49.83 gbp I then converted the USDC to bitcoin when I dispose of the bitcoin which cost do I use to work out any loss/gain for self assessment/capital gains tax do I use the £50 or the £49.83 cost?

Hi,
The £50.
Thank you.