John - Local Authorities may have 'special powers' with regards 'vacant properties' but that is more to do with ensuring housing stock is occupied for example. They would still be reliant on the legal owner, registered or unregistered either way
Any wider powers would be with regards their own land/property and not someone else's in my experience
If a legal owner dies intestate then 'bona vacantia' can play a part -
Bona Vacantia - GOV.UK