This sometimes happens when you view or buy an item from an American seller, then everything is shown in US $$'s.
You need to log out completely, even shut your device down. Start up, and log into your homesite, https://www.ebay.co.uk
Clear cache and cookies, too.
Everything will be back in Sterling Pounds. If it doesn't switch back, just refresh your page a few times, that will kick start it into showing the correct currency.
@jomack-8771