Does anyone know if it is possible to use the "pay cash, upgrade with avios" option over the phone for open jaw (or as in my case double open jaw) routings?

The online booking engine only seems to be able to handle simple return bookings (even just a return trip but through LHR gives me the incredibly annoying "error - try again or if it still doesn't work you might wish to try again later" message).

I am looking at PEK-LHR-CPH (WTP) // LHR-CTU (WTP upgrade to CW using avios).