You should check the other guidance available on GOV.UK from HMRC as Brexit updates to those pages are being prioritised before manuals.
Paragraphs 9 to 11 of FA03/SCH2A concern pre-completion transactions that are free-standing transfers.
Paragraph 9 concerns the transferee. It adds any consideration given for the pre-completion transaction to the consideration otherwise given by the transferee. As for assignments of rights, the vendor for the transferee’s acquisition is generally the original vendor, subject to certain exceptions (see paragraph 10).
No special provision is made for the transferor. If the free-standing transfer is a subsale, then the original contract will be completed (or substantially performed) so the transferor will be the purchaser under a land transaction as normal under section 44. If the free-standing transfer is a novation, then there is no land transaction under which the transferor is the purchaser, so the transferor has no need to file a return or claim relief.