How many packages does customs actually check?

My pet gerbil's brother may or may not have ordered relatively large quantities of pharmas into the country, declared as something else by the sender, fake return address, fake value (under the tax limit), and those packs if they existed would have gone through without a hitch.

Don't forget too not only did the UK leave the EU but also is not in the Schengen area. So full customs checks are in place.

Still easy to smuggle in drugs let alone a painting.

China when sending phones and other tech also falsely declares everything. If you go on AliExpress the sellers openly advertise in their listings how you won't have to pay import duties.

Basically no one cares.

Also, if you buy stamps with cash and deposit the packages at different post offices, what's the odds of it linking to you?

Food for thought.