Correct,the PIAT needs deactivating, you need to contact your local Firearm Enquirey Officer & arrange to surrender it, they will hold it until you can arrange for the deactivation work to be carried out by a section 5 armourer & submitted for proofing. You can not carry out the deactivation work yourself.
In it's current state is a LIVE weapon & illegal to own without being registered on a section 5 FAC.