My understanding is that goods in free circulation in the EU (in other words made in the EU or EU customs duty paid if made elsewhere) are not subject to customs controls when moving from one member state (Czech Republic) to another (UK), but I guess it's always possible that UK customs/border staff might open things to check that they are what they claim to be.

It's up to you what you do, but I would say the probability is that if the package was opened in transit it was most likely opened by someone in border control who didn't even know what they were looking at or what it was used for.

That the package was opened only represents a risk to your use of the Trezor if it was opened by someone who knew what it was and had the means and skill to compromise it. You can't entirely rule that possibility out but I think it's unlikely.

This issue is really just part of the whole 'how can I be sure my bitcoins are safe?' issue.

I'm a Trezor owner myself, but I don't trust all my BTC to Trezor. It's just one of several options I use. It's very convenient for having a very secure means of storage that I can also spend from. But I don't use it for all my BTC. The point is not to expose all your BTC to a single point of failure.

Bottom line is that however you secure your bitcoins you cannot be 100% sure they are safe. You have to be trusting some piece of hardware or software, and trusting that anything delivered to you has not been back-doored in transit of at the manufacturer. How do you know the sealed Trezors are not compromised? Haven't been opened in transit by a malicious third party who has the industrial means to reseal them to look untampered with?

No matter how you create e.g. paper wallets or offline signing set ups, at some point you have to exercise trust, unless you are building everything (hardware and software) yourself from raw materials, which of course no one can do.