Your sold  items are here:

https://www.ebay.co.uk/mys/sold

You can cancel a transaction via the dropdown actions menu alongside the item and use the Second Chance Offer link to offer to the underbidder.   If they haven't paid if you can wait 4 days you can then use the unpaid option to cancel.

If the SCO link is not there (apparently it may disappear after cancellation) click on the original listing on your sold/orders page, then click on where it says the number of bids in the middle of the page and you'll see it alongside the bidders' IDs on the next page.