B777 Captain reported encountering wake turbulence in cruise flight at FL320 in trail of a B747.

Narrative

About 1 hour after takeoff; we were over Scotland at FL320. FO was flying and the autopilot was on. I was monitoring. It was smooth with no turbulence in the forecast; the seat belt sign was off. There was an aircraft on the TCAS about 11 o'clock and approximately 18 miles in front of us at FL340. We were radar contact with Scotland Control. There was a quartering left headwind at 81 kt. Unexpectedly; we encountered moderate turbulence. I immediately turned on the seat belt sign and told the flight attendants to be seated. The FO said it was wake turbulence and I agreed. I told Scottish Control we needed a descent immediately. They gave us FL270. As soon as we left FL320 the turbulence stopped. Scottish Control called us again and I told them we encountered wake turbulence. They asked if we wanted to stop at FL300 and I said yes. I waited until I was sure the turbulence was over; then had the flight attendants check in. The Purser told me there were no injuries but that some of the meals fell to the floor and had to be thrown away. Once the aircraft in front of us had moved away from us we climbed back to FL320. It was smooth.