I did a simelar thing a few years ago flying from England. I needed to fly LGW > PDX. The cheapest fare at the time was around $900, but the fare to EWR was $450. I bought a r/t LGW > EWR, and used 25,000 miles to book a seperate ticket from EWR > PDX watchting the connection times etc. When I showed up at LGW with 2 seperate sets of tickets I had no problems, the agent even checked me and my baggage all the way through to PDX.