I'm in the UK and can assure you that all they will do is seize it.
I wouldn't be so sure tbh.
Chances are that they will not prosecute you for the Kratom due to the reasons AT has stated. If you incriminate yourself however then that
could be a different story. They do so love an importation case, and the penalties are significantly higher than for a domestic purchase. I will say that I'm unclear regarding the status of kratom here in uk.
There is also a
possibility that they will obtain a warrant and search your house. Make sure it's clean of anything illegal. Assume they will seize your computer/phone and act accordingly. They may or may not do that - nobody can give you a percentage chance. It's a possibility though. It can be random and there are hundreds of variables...where you live, if the pigs are busy or not, if they got laid recently and so on
It's truly unpredictable. I have heard of (true) cases where someone got a loveletter from Border Force (a seizure notice) for 50 ecstacy tablets and were never visited by police, and also of someone who got their house ransacked and devices seized for importing 7g cannabis, wtf. 100% true. Go figure.
Do not talk to the police apart from giving name etc without a solicitor meeting. Whatever the pigs tell you, there is nothing to be gained from talking to them.
Never order anything illegal from abroad again unless you can use a drop unconnected to yourself. Fuck ordering anyhting from abroad, just too stressful for me.
Loose lips sink ships. Good luck (chances are you'll be ok but who knows. Use it as a learning experience though for sure)