There is currently an issue with Simple Delivery not offering Custom Postage to items like yours, that is, those exempt from Simple Delivery. It's taking a long time to fix, and frustrating for many sellers.
Thankfully, there's a good workaround.
From the link below, Go To Solution and follow Rick's answer how to list your item.
https://community.ebay.co.uk/t5/Seller-Central/how-do-you-list-anything-bigger-than-61cm/m-p/7882638...
@jacksgrandadken
