At the weekly chat with community staff they were asked:
Will we still be able to sell internationally after Simple Delivery becomes mandatory using labels purchased ourselves from Royal Mail for example?
And the reply was:
You will be able to send international items as normal but SD is available for GSP items for the first leg of transit to the UK Shipping Centre.
So that sounds like we don't have to use SD for international sales as long as we don't use GSP. Not sure how that will work in practice though.