Are there currency conversion fees when I sell?

All sales are completed and settled in GBP, so there's no currency conversion involved in the sale itself, regardless of where you're based.

If you wish to then withdraw your proceeds to a non-GBP bank account, prevailing currency rates will apply.

If you're investing from outside the UK, see Can I invest in wine if I don’t live in the UK? and What currencies can I trade in?