An oak beam or wooden mantle must be a minimum of 18 inches or 450 mm away from the top of the woodburner ie the underside of the beam must be at least 18 inches or 450mm above the stove top.
A wooden beam is a combustible material. The wood burning stoves and multi fuel stoves must be a minimum of 18 inches or 450 mm away from a combustible material such as wood.
Many people, instead of inserting a beam into the wall for a mantle or lintel, they place the beam above the stoves like a shelf.