ACPO recommendation in their recommendations from 2015, Police want to make it a criminal offence to be in possession of a deactivated firearm that has not been deactivated to the CURRENT standard. Anyone with an old spec deactivated firearm will have to have it done to the current standard if they wish to keep it.
So don't expect any favours on de-acts once we're out of the EU, its one of many items that ACPO/Police want to change/tighten up/ban etc.,etc in the coming years until every firearm of any description is banned in UK.