I know this is an old thread, but it also pops up in Google if you're looking for information about this topic (as I was) so might be worth just adding my experience here:
Ordered parts (mag and a replacement bolt) from WGC that shipped 27th and arrived in the UK a couple of days later. I got the form everyone else got, filled it out with just YES to RIF parts. Didn't fill out anything else other than the extra text bit where I wrote that these parts don't alter the appearance (not actually sure that matters in retrospect) nor are they an RIF, so no extra information is required. I also obviously filled out name, DOB, tracking number etc and 'signed' at the bottom.
It entered customs and then was cleared (according to the tracking) the same day, then asked for import fees, then was delivered a few days later. It also had the bright orange AIRSOFT sticker on the box, so I don't think it was missed by customs or not looked at. Box was otherwise unopened.
@PewPew4U obviously feel free to ignore this, but what happened after your AUG was seized? Were you able to get it back? Or was that just the end of it?