I think the first use in a British context, of the American phrase paying down, to mean reduce mortgage debt was in the first TV election debate when David Cameron used it. I suspect he picked it up from the American advisors he was consulting.
Nick Clegg used it in his last email to Liberal Democrats.
Now Larry Elliott.
All the Building Society and Bank web sites that I've checked are still using reduce.
It will be interesting to see how the phrase progresses. Will it stay confined to politicians and journalists, or will it get taken up by the British financial services industry and the general public?