You can only opt out of SD if you're selling an extra large/heavy item or a letter size item under £10/100g.
If you are selling extra large item when you click on the packet size box during the listing process you will get the various size options in a pop-up. Scroll down to the bottom of the pop-up list and you'll see a link that says:
"Looking for heavier or larger package options? Switch to custom postage."
Then you'll be able to set your own postage.
For letter items check the eligible sub-categories in FAQs on here:
https://www.ebay.co.uk/sellercentre/postage/simple-delivery?targetgroup=C2C
click on "Which listings aren't required to use Simple Delivery?"
If neither applies you're stuck with it.
