4' x 4' x 8" Router Build

[JonnyFive]

Entirely normal, if the cost is over £135.
Under £135, the seller should be VAT registered, and collect the VAT on the UKs (and I think it's also came in for the EU) behalf. If you paid through an online marketplace (aka eBay, Aliexpress), then they have to do the VAT collection.

As for the one you didn't have to pay, give it a month. Some couriers deliver, then send you the bill. Others bill you before they'll deliver.
You might have been lucky. I know I've never been billed for some orders in the past, but generally higher value stuff doesn't slip through the HMRC net.

Good to know, thank you!! I’ll brace myself for the upcoming bill on the BST delivery