Theroux on Britain: fogged up by the writer's perceptions; The Kingdom by the Sea, by Paul Theroux. Boston: Houghton Mifflin Company. 320 pp. $15.95.

In this journey around the coast of Britain, the melancholic character of expatriate American travel writer and novelist Theroux stands between us and that nation. The challenge facing the reader is to discern how much of this book is really about Britain and how much is about the mind of Paul Theroux.

From Page 1, Theroux (author of ''The Mosquito Coast'' and ''The Old Patagonian Express'') makes it plain that he is a hard man to please. He shrugs off Samuel Johnson's admittedly 18th-century warning that ''a man who is tired of London is tired of life'' and says that he is tired of a whole list of things about the British capital, where - nonetheless - he has chosen to live for the past 11 years.

But he is curious about the rest of Britain, and decides the way to discover the country is to travel around its coast by train, bus, and foot.

He chooses to start by boarding the 11:33 train to Margate on the Kent coast on the May Day holiday, 1982, knowing full well that relatively harmless ritual fights between gangs of youth take place every year on that day on the Margate sea front. And yet when rowdy youths board his train, he expresses surprise and outrage.

From this clearly calculated, action-packed beginning, it becomes apparent where Paul Theroux's talent lies in this book: he excels in descriptions of the negative. He is depressed by the many working-class holiday resorts for which Britain's coast is famous, and yet neat and tidy middle-class houses seem to bother him more. Throughout the trip he chooses to stay in very cheap hotels. Yet the most profound conclusions he passes on to us is that cheap hotels are really not furnished very nicely.

In the course of his journey, Theroux admits he comes across beautiful scenery, but he doesn't dwell on it much. It seems more difficult for him to find adequate words for beauty.

When he crosses the Irish Sea to Northern Ireland, the book briefly comes alive. The agitated atmosphere of Ulster provides the kind of tangible misery into which Theroux can dig his descriptive teeth:

''I had never imagined Europe could look so threadbare - such empty trains, such blackened buildings, such recent ruins: DANGEROUS BUILDING - KEEP CLEAR. And bellicose religion, and dirt, and poverty, and narrow-mindedness, and sneaky defiance, and murder, . . . and empty stores, and barricades, and boarded windows, and starved dogs, and dirty-faced children - it looked like the past in an old picture.''

That Northern Ireland certainly exists. But those who know Ulster well are aware of another side to life there, to which Theroux seems oblivious: War can bring out the best as well as the worst in people, and family life and human relationships take on an added importance in a place where the daily round is frequently interrupted by violence.

This is a book that will slide neatly into place in the matrix of received opinion about the hard-pressed United Kingdom of the 1980s. It has been given a lot of publicity in both the United States and Britain, is doubtless selling well, and will stand prominently on many shelves as the definitive word on Britain - until perhaps an even more distinguished writer than Theroux tackles the subject.

To be fair to the book, it would be foolish to deny the obvious truths Theroux sometimes expresses: that mass unemployment has left its mark on Britain; that the industrial revolution in all its many incarnations has left the landscape scarred; that the British people, like those of any nationality, can be infuriating in a multitude of ways.

Theroux himself says that the British, particularly the English, are a secretive people: They protect and hide what is most important to them. To get to the true, secret Britain was the challenge that faced Theroux. He may have traveled around the edge of the United Kingdom - but he got nowhere near its heart.

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