I'm trying to book : SIN-BKK (CX), BKK-LHR-GVA (BA). Based on the distance, this one-way ticket should be 70k + taxes.

However, the agent on the phone argues that there are 2 transfers (less than 24 hours in BKK and LHR), hence it is not allowed. He argues that there can only be 1 stopover OR 1 transfer on a 1-way award ticket. I find it hard to believe and have never found such rules in the T&C.
I have asked to talk to a manager who basically says the same but are not capable of telling me where are these rules written.

can anyone help ? is that true?
Thanks.