I was able to book a direct LHR-CMN on RAM with Avios online but struggled to do the return leg - only mixed carrier (no directs on RAM) bookings were available but always ran into "there is a problem doing this online" message. In the end I called up to try for CMN-LYS-LHR (second leg on BA) and even the agent couldn't price it there and then and called back a couple of days later with the quote, then went through fine.