I have seen no discussion on these types of Trust.
a) National Savings Certificates held in trust (bare trusts).
b) Shares held in trust for minors. (bare trusts).
There must be millions of these in existence which there can be no good reason to register but as far as I can see have not been excluded.
I was a trustee of a bare trust for a minor. The trust fund consisted of various shareholdings. Even though the shares were sold (in May 2021) and the proceeds distributed, the trust still has to be registered. There is no exemption.
As far as I can see there is no exemption for trustees of bare trusts holding National Savings Certificates.
As to why these need to be registered …
The registration requirement is a consequence of anti money laundering legislation (not tax legislation) and it is possible that bare trusts could be used as part of a money laundering strategy.