The biggest change is that now the person shipping (or a marketplace if they use it) has to be VAT-registered and pay VAT directly to the government, rather than it be handled by the courier/shipping service who would collect it directly from the recipient before releasing the package to them. I think it's a pretty significant change to how third countries work, especially if they only occasionally ship to the UK.

The EU are adopting the same rules in July, and I'm not sure why we didn't harmonise the introduction date with the EU.