Do they actually check your stuff?
As someone who has annoyed border guards all over Europe, I can tell you that luggage checks on non-EU borders are pretty routine. It depends on the exact border in question, but for instance, on the Terespol-Brest train, you'll have your luggage checked on entry to both Poland and Belarus. They're looking for different things (Poles for cigarettes and booze, Belarusians for drugs), but the checks do exist. Same story on the Hungarian-Serbian border, where the Hungarians are looking for cigarettes while the Serbians are checking 'in general'. I've even had a thorough exit control by the Serbians after they found my passport to be very suspicious due to all the stamps from random Serbian crossings. Plenty of controls in both directions between Poland and Ukraine too.
Even Ceuta (Spain)-Morocco - the Moroccans were stopping random people to have a look at their baggage, while the Spanish were checking every car carefully.
Or from last winter in Cyprus, the Cypriot customs would check people's bags when they entered at the Ledra Street/Lokmaci checkpoint, though the North Cypriot customs couldn't care less what you bring in. The Cypriot customs are a bit more relaxed elsewhere, though they did ask my 3 year old what his name was at one crossing (Deryneia/Kato Deryneia).
Even at the UK/North Cyprus checkpoints, they were checking my boot every single time when crossing from North Cyprus into the UK, though they were pretty much ignoring cars leaving towards North Cyprus.
Same story on Croatian-Bosnian and Croatian-Montenegrin borders - while the Croatian customs rarely check the car physically, they always ask "anything to declare?". I
You can even encounter these controls on internal EU borders. From personal experience:
Gibraltar-Spain by car: random boot checks.
Gibraltar-Spain on foot: luggage frequently x-rayed, sometimes personal checks too.
Spain-Gibraltar by car: usually a look in the car by Gibraltar Customs.
Spain-Gibraltar on foot: occasional luggage searches, though it seemed more intelligence-led than anything.
Ceuta-Spain by ferry: all luggage scanned on entry to mainland Spain by Customs, questions asked to some people.
Spain-Ceuta by ferry: Spanish Customs occasionally checking people, though not systematically.
Slovenia-Croatia by car: occasional checks of the car by Croatian police, once had a thorough check when leaving Slovenia by Slovenian customs.
Croatia-Slovenia by car: occasional checks by Slovenian customs. I've once had my documents inspected very very closely after having crossed the border for about the 10th time that day. I also got stopped very close to the Croatian border by the police who wanted to know what I was doing and where I was going, and they did a thorough check of my documents. They were fine with the explanation though (interested in geography, wanted to look at the border at different points, well equipped with maps showing the precise location of the border in order to avoid accidentally crossing it, promised not to get too close to the border).
Of course, talking about the UK, I've had customs control at the old Arrivals terminal in Dover, though they were pretty hopeless: they didn't pick up on the rather large amount of French fireworks.
So yes, customs controls are pretty routine. I've even seen controls between Denmark and Sweden, and even Sweden/Finland have maintained the old border crossings for random checks. The Norwegian border is quite tightly controlled as well, despite being a Schengen border.
Talking about juxtaposed controls in general: St Pancras is going to be a huge bottleneck unless they do something about it. There will be a lot of non-EU citizens that have to be checked for their health insurance, source of funds, purpose of travel, etc etc, and there's simply not the space there to do it. I think they need to consider reconfiguring the arrival lounge to avoid all that wasted space, especially as they really only need a few small rooms for so-called 'secondary' checks as well as a couple of luggage scanners. One solution could be to do the customs controls at the immigration control, or use technology to automatically 'tag' anyone that might be worth a secondary inspection in the UK. It's not impossible to simply use facial recognition technology and some automatic gates in St Pancras on arrival, with anyone pre-selected for customs control to be automatically directed away from the exit. They could introduce the requirement that customs declarations have to be made before departing from the EU, which would reduce the need for space even further in St Pancras.
