Julie - form JO would only be used with form AP1 if there was another transaction involved. It doesn't read as if there is.
Severing your joint tenancy/splitting the beneficial ownership is usually achieved through form SEV -
Joint property ownership: Overview - GOV.UK
We don't record/register the 50/50 or 76/25 split so worth checking to confirm that the register doesn't include a form A/joint ownership restriction first. If it does then we don't need to know about the updated split
Search for land and property information - GOV.UK
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