The buyer pays postage. Payment goes to ebay who then holds money until after delivery. If the postage option you want is not there you can opt out of their Simple Delivery process. You do that during the listing process. Look for the Custom Postage option and if it's there select that. If not click on the postage options icon on the right of the postage section and remove details of parcel weight and dimensions. Then you should be able to add what postage you want.
Here is information about the Royal Mail collection service:
https://www.royalmail.com/collection
If you buy the postage outside of ebay, not using a label through them, you must remember to mark dispatched and add the tracking number manually via the dropdown menu arrow for the item on your orders page.
