It would be CGT, because you were not in the business of trading in
domain names for profit. This is much a similar situation to when
distinguishing gain from income in the case of someone buying a
property, doing it up, renting it out for a while, and then selling
it. If he does this, the rent would be income and the profit from
sale would be gain. But if he were basically a serial restorer,
buying, doing up, and re-selling, he'd be considered a property
developer and profit from the sales would be considered income
rather than gain.