If it's the Brompton it's recorded as sold. Whether it was the Brompton or some other item, you need to give the buyer four days to pay and can then cancel for non-payment, see: https://www.ebay.co.uk/help/policies/payment-policies/unpaid-item-policy?id=4271 The buyer will be penalised with an unpaid item strike by eBay.
If it was for a different item and you have already jumped the gun to cancel the sale I hope you gave the reason as "buyer requested to cancel". Making a seller-initiated cancellation would place a defect on your account.
Sellers are not allowed to leave anything except positive feedback and positive comments for buyers, however badly they behave. Don't try to get around this or you could get a defect from eBay or feedback violation policy.