Really no requirement to register?
'BASC's Matthew Perring added: "There is no requirement of 'registration' for deactivated firearms, only 'notification'."'
That's a purely semantic point that can't be relied on to escape prosecution.
The Firearms Regulations 2019
Notification of deactivated firearms
3.—(1) Subject to paragraph (2), a person commits an offence if—
(a) the person is in possession of a deactivated firearm;
(b) the person does not give notice of the deactivated firearm to the appropriate nationalauthority in accordance with paragraph (3), and
(c) where the deactivated firearm has been transferred to the person, the person whotransferred it has not given notice of the transfer in accordance with regulation 2(notification of transfers of deactivated firearms).
Fortunately, Regulation 3 has been provided with a transitional provision applicable in the case of extent ownership:
4.Regulation 3 (notification of deactivated firearms) does not have effect until 14th March 2021 in respect of deactivated firearms which came into the person’s possession before 14th September2018.