Ebay UK under "Postage Set Up" as you work through listing your item.
It is sometimes half-hidden by pop-ups like " Postage is easier now" and you may have to select several times as you go through the process. They don't make it easy but eventually you'll be able to put your own postage price and carrier under "Custom Postage".
I mostly use "Sell Similar Item" when I list and just alter the Item Title, photos and Description so perhaps it is deifferent starting from scratch.