It appears that Soames of special branch gave the authority to destroy specific exhibits in 1996.

Which could mean:
1) Special Branch is in charge of old evidence and has to sign off before any destruction can take place and thus peopel have to request permission from them
2) Soames requested the blood samples be destroyed for some reason such as to make room for Special Branch evidence.
It is unclear whether that means Soames just signed off or requested it be destroyed.
The letter asks for information that would explain which is the case and why but those documents have not been posted to leave a vaccuum in which unsupported conspiracy claims can be alleged and the actual evidence to refute those claims kept secret.