Ok, here is how I understand it... :-D
The EU Regulation has become part of national law of all Member States (and that includes the UK) the day it was published (April 8th 2016), without the need of having to implement it. So strictly speaking it has been illegal to sell, gift etc. any deac that doesn't comply with the rules states in the Regulation since then.
Practically speaking the Regulation as-is will be impossible to enforce ofcourse, and efforts are now being made to have pre-Regulation national standards of deactivation that are of the same level as the level prescribed in the Regulation, accepted as equal so that any weapons that have been deactivated under that older national standard will be accepted by the other Member States as equal and as such comply to the Regulation. In many cases it would technically impossible to upgrade to the new deac standard anyway.
Strictly speaking I think that means that since April 8th last year it is illegal to sell any deac that doesn't comply to the new Regulation, but you are allowed to keep them if you already own them. Ofcourse this is impossible to enforce and police know that, so it is unlikely it will have any consequence for the moment at least if it happens anyway. That doesn't mean it isn't against the law though, so use your own judgement.
There is a decent change UK new-spec standard will be declared equal to the Regulation's standard which will be great news (then you can buy, sell and export them without problems) but I'm pretty sure that won't be the case for the old-spec standard. That means those will be legal to own, but not to sell etc. unless brought up to the new standard. The normal consequence will be a underground market...
In short, the whole thing is still a work in progress and nobody really knows how it will finalize.