Have they sent you notification that it has been seized? It may take a while to get you if they have. How long has it been stuck?
If they have seized it and the FPS is not written on the box, they may chrono test it (Which they have done in the past) and destroy it if it fails the chrono or if they see that it says 390 FPS anywhere on the packaging or the documents they will destroy it without testing, there is also a slight possibility that you will be fined and/or prosecuted (it's written into the law, but there has been no known precedent that I am aware)
They will not send it back if it's a section 5 firearm, they will only destroy it.
Why on earth did you order it without getting the retailer to lower the velocity? Why import a section 5 firearm?
Do you have a UKARA number? (It won't help so much, but just curious).
As with anything, the onus is on the buyer, not the seller to know. The seller cannot possibly know the local laws of every country, but the buyer should definitely know the law of their own country.
The buyer would lose the dispute.