This one is easier to understand than you may think, it is EU law that all EU members will recognise the standards of all EU proofhouses.
However, the UK government feal that the foreign standards arn't as tight as ours and therefor don't recognise them within the UK. This dosn't make it illegal, it just means that the crown would have to take any case take to court, and at the moment I think they know they will lose by the defence representitives simply refering to the EU standards, if they won, they would be breeching EU law. Thats why they will not take it to court as that would give a difnitive answer and the Crown would prefer it to be a grey area. Companies like Rusmilitary are just pushing the Crown and at the moment they turn a blind eye.
I have a Belgian deac STEN Mk3, its the same spec as pre 95 but I've no idea when it was done. I looked at buying a Browning 1917A1 from France, the proofhouse there (French government owned) told me in responce to my question 'Can I Import this to the UK' that I would need to ask one of the UK proofhouses, the Birmingham house stated it was not legal, but again, they are happy to say such things as they are protecting their business. Both UK houses are commercial companies and would not like to see us buy from their competitors abroad, that really would be the end for one of them. When you think of it as each proof costing £60, and 1000 guns are proofed as deactivated every month, that £60,000 a month they stand to lose if we buy abroad.
What we need is someone with money, influence and big cahoonas to challenge the Crown/Government and get a diffinitive decision.