Why would you want to convert the title?
The leases registered on a cross-lease title restrict your use of the property. For example you can’t construct a standalone garage or shed on the property without the consent of the neighbouring flat owners. Making external alterations to the dwelling requires the flats plan to be updated to record the new flat outline. If any of these requirements are missed, this could lead to issues when you come to sell the property in future.
There are no such restrictions with a fee simple title. You can make alterations without having to update the title.