You could check on the Evri website if you can book a collection with them but eBay say collection is available with Simple Delivery through Royal Mail. So you'd have to cancel the current transaction (via the dropdown arrow for the item on your orders list - a refund will be issued). Tell the buyer what you're doing.
Then opt out of using Evri here:
https://www.ebay.co.uk/ship/prf
And relist. Once the buyer has bought and paid again, after you've generated a prepaid label on eBay you will be able to book your collection on the Royal Mail website.