BestHeating would recommend leaving 50mm above, and 100mm below your radiators, to make sure they offer the most efficient and effective means to heat the room they occupy.
Leaving clearances of this size at either end of the radiator will guarantee that the heat generated by them can travel freely around the room, and heat the space to the best possible effect.
Our Advice Centre blog, Where to Position a Radiator, offers a more detailed insight into the best place to install your radiators.