That sounds perverse, frankly, especially as it's a manual process so I can't see that there would be anything to stop them just booking you 2 seats for the points price of one (or minus 150k off 2 x return price).

I wonder if you just got an incompetent agent (though they are generally very good at the Cardiff call centre)? I was allowed to use my voucher to upgrade by 2 cabins from Y to UC recently, so I paid 15k points plus taxes for 2 x UC MAN-MCO. If they have that kind of discretion, I'm sure they can also apply it as a 241!