@John *******
Sorry, but you missed to take notice of something really important, when you applied for the 1-year multi entry Non-Imm-O family Visa in the E-visa system of the London embassy.
The 365-days multi entry Non-Imm-O Visa (both family and retirement) got discontinued by October 30th, 2023, in all over the World's Thai embassies and consulates
(except for reason “married to a Thai wife” in Savannakhet, where it remained to be still available until now, but yet to be seen if the still offer it after January 1st, 2025, when the e-visa system will be introduced in Savannakhet)
Only the 90-days single entry version remains available.
If you need a multi-entry that is valid for one year, you first must enter with or apply inside Thailand for the initial 90-days Non-Imm-O Visa,
And after you have fulfilled the financial requirements, apply on Immigration for the "1-year Extension of Stay".
Only then, for this extended 1-year stay permit, you are able to buy a multi re-entry permit for 3800.- THB