Unless anyhone else has experienced this then only Keston might be able to give a reply as to why its U12.
I had a similar "wrong" fault prefix on a fault code on another boiler and the maker decided that it might be a faulty or early pcb but the meaning of the code in relation to the fault would be the same.
Regardless, you are going to need a competent gas registered engineer and in view of the high spares cost, a British Gas fixed price repair will probably be the best option for you.
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