Does anyone know if this is England-only, or if it affects Wales/Scotland/Northern Ireland too? How does the opt out work if you've been in multiple areas/GPs/etc.?
I think government assuming ownership of [parts of] persons is a bad thing, yes.
IMO it makes logical sense to either not assume ownership of anything a person had in life, including their body and allow their family/executors to look after the remains of their person , effects and assets [the status quo]. Or you should say that ownership is void on dying and all assets, including one's body should pass to the state.
It would make far more difference for the state to assume ownership of the things a person had, houses, bank accounts, than it will make to only assume state ownership of a persons body post mortem.
There doesn't appear to be a consistent argument to compel one to take this middle ground.
At least here in Germany, we had some scandals around organ transplantation. It's big business. And there is a difference between recovery treatment and organ preservation treatment. The emergency team could be under economic pressure when doing a triage.