tankie , Bren barrels were 'done' at the London proof house about 7 years ago as per your spec with an 8mm slot milled up the length of the barrel.The gas regs were pinned and welded in situ.
They are not 'deac barrels' though, but have been deemed as "scrap-not suitable for repair" As they have been classified as scrap by the proofhouse I would suggest this is the way to go rather than to get them to class them as deactivated.

ATB Kevin