There's a simpler way to resolve this.
Someone get a correctly marked and genuine ex
UK
MoD L59A1 entered on their FAC. Then, when it's due for renewal or before, ask to have it removed from the FAC as, after due consideration and technical advice, it is not a firearm as defined in law. Present the rifle, together with a copy of the UK MoD spec plus a report from, say, the person who drew up the spec in the late 70's together with his qualifications and experience in this particular matter/field. That person may well be invited to present your case in a robust and technical way for the price of his lunch and mileage
They might huff and puff a bit but after learning that another Force have allowed such a beast, they'll remove it.
There, that's food for thought isn't it. I have had TWO such beasts removed from my FAC in the past. Remember the phrase. Where there's a will, there's a simple way