And so the predicted apocalypse begins - it's more difficult to buy spare parts for your bicycle now.
Joking aside, the rule seems ridiculous. Why should companies in the USA, India, China, that sell the occasional small item to someone in the UK, need to register for UK VAT collection? And what does the UK think it can do if the company just ignores the rule and sends the parcel anyway?
If every country implemented such rules then it would make international trade a nightmare: "We've just sold one of our T-shirts to a guy in Bengal. Does anyone know how to register for paying tax there? Who speaks Benagli?"
Surely it's better to just do what non-EU countries have been doing for years already. Don't charge any sales tax (or sellers often still charge the sales tax for their own country if they just sell the occasional item overseas and can't be bothered repricing). Then it's up to UK customs to intercept the parcels and charge the receiver VAT and import duty. Sometimes they do this, often they can't be bothered for smaller cheaper items and you, as the buyer, get away without paying the extra charge.
Companies that sell loads of stuff to the UK probably wouldn't mind registering, so that they could offer their UK customers a cheaper faster service with the customer not running the risk of being hit with extra customs and handling charges - this is going one (very small) step towards setting up a UK-based daughter company to handle your sales there. And of course, companies that sell tons of stuff into the UK will already have UK-based distribution agents, or UK distribution offices of their own.