I've been through this several times, the last one was just last week. You've already been given the advice you need, but I thought I would add to this thread...
They can absolutely seize it under Section 36 of VCR Act: "(7) A realistic imitation firearm brought into Great Britain shall be liable to forfeiture under the customs and excise Acts."
The defences in Section 37 apply when you are charged with an offence, which Border Force aren't doing. In reality they will release it to you provided you have a defence, at least they have in all of my seizures going back 10+ years.
The letter does cover it, but if it isn't clear, you want "Restoration". Write to them at the address provided, make sure your name and address is on the letter, provide them with your defence, IE, UKARA number and the site you play at. They have access to UKARA, which makes me think maybe it wasn't on the box, or wasn't clear. I would also advise to add your email on it to and ask them to contact you that way if there are any further issues, you don't want to be going back and forth by post. You also have to provide an invoice or receipt of the goods, again this should have your name and address on.
Failing that, Border Force do take bribes; Send them a box of Ferraro Rochettes and all your problems will be solved.
Good luck.