Most British travel writers head south for a destination that is hot, exotic, dangerous or all three. Harry Pearson chose to head in the opposite direction for a country which is damp, safe and of legendary banality: Belgium. But can any nation whose most famous monument is a statue of a small boy urinating really be that dull? Pearson lived there for several months, burying himself in the local culture. He drank many of the 800 different beers the Belgians produce; ate local delicacies such as kip kap (jellied pig cheeks) and a mighty tonnage of chicory and chips. In one restaurant the house speciality was ''Hare in the style of grandmother''. ''I didn''t order it. I quite like hare, but had no wish to see one wearing zip-up boots and a blue beret.'' A TALL MAN IN A LOW LAND commemorates strange events such as The Festival of Shrimps at Oostduinkerke and laments the passing of the Underpant Museum in Brussels.No reader will go away from A TALL MAN IN A LOW LAND without being able to n