On here:
https://www.ebay.co.uk/help/selling/posting-items/setting-postage-options/simple-delivery?id=5575
It says:
Restricted or prohibited items - The following items aren't allowed to be posted with Simple Delivery: Valuables, such as jewellery, watches, and precious metals, including, but not limited to, jewels, watches, gems, precious metals, real pearls, silver, gold.
So you should be able to opt out of SD. If it won't let you the alternative is to list with free postage, factor that cost into your item price and buy your own postage outside of eBay.
To get the buyer's full address and mark dispatched click on Get prepaid label > Print label for the original Simple Delivery label. It will remain unused and you can claim a refund for it here:
https://www.ebay.co.uk/claims/simple-delivery/refund/seller
The option to change the tracking has been removed but you would be able to add the new number to a "not received" case if that were necessary. Without tracking on ebay the payment may be held for 14 days.