Hi Ian - whilst you can register first and then transfer that would be a very unusual way to do things
Better to register for the first time based on the Transfer from the executor to whoever as a single application
It would not be a TP1 as that is used to transfer a part of the property such as part of the garden
The legal ownership can only be transferred as a whole and if the executor is transferring the legal ownership to say a beneficiary and someone else then form TR1 can be used.
The Transfer will make it a compulsory first registration application
HMLR Guide: Start - External · HM Land Registry
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