UK customs will only seize if it is picked up on a spot check, not everything is checked. If it is seized then you will get a letter asking for defense. If it was me I'd get the store to include a documentation wallet on the box with the shipment that included my phone number, email address and reason for defense.
In regards to customs and UKARA I read somewhere that there's no mention of it in the UK Border Force Operations Manual, however others have stated they use it when the RIF goes to their Birmingham site for processing. Who knows...