Thanks @BiancaBlake , we’re making progress. It seems the Shopify information I linked to is wrong. Here’s something understandable from gov.uk itself:
There’s no threshold if neither you nor your business is based in the UK. You must register as soon as you supply any goods and services to the UK (or if you expect to in the next 30 days).
My solution is going to be to not sell to UK customers, so how to do that will be my next question. Imposing this burden on small (<£135) sales into the UK, and having an easier process (bill to customer) for sales above that threshold seems ridiculous to me. Maybe it’ll be changed in the future.