Like so many people who ask questions, you haven't given any of the information that may help to answer. Are you using the mobile app, or a PC (or Mac)? What browser are you using?
If you're using a PC, try opening a separate tab in the browser, go to PayPal's UK site and log in. If you then try again to pay for the item through eBay checkout, it should recognise that you're already logged in to PayPal and enable you to complete the purchase.
If this doesn't work, try asking again with more information.
