ISLAND FORTRESS The Defence of Great Britain 1603-1945 By Norman Longmate (Grafton, $260) THIS volume picks up where Defending the Island, also by Norman Longmate, left off. It is an extremely good read, even at 600 pages. In fact, had I been given this when I was 19 or 20, I would not have gone through life with only the vaguest notions of British history. Now, thanks to the narrative style, one ruler or one event, leads logically into another. We begin with King James, already King of Scotland but invited to London as the first Stuart king. He did not impress everyone: ''King James was the most cowardly man that ever I knew,'' wrote one official. James was positively averse to military matters. But by the time we get to Charles I and the English civil war the emphasis shifts to weaponry and the New Model Army of Cromwell. A new type of firearm, the fusil, was introduced (hence Fusiliers) along with the hand grenade, to be thrown by the tallest men (hence Grenadiers). Cromwell's rule was strict but, on the whole, fair. But when Cromwell died, a virtual benevolent dictator, his son proved insufficient for the task. At last Charles II's time had come after years of hiding up oak trees and wandering the courts of Europe looking for hand-outs from whichever country happened to be at war with Britain at the time. It was never difficult to find some nation vowing to bring the British down, once and for all. Throughout the 17th and 18th centuries there was a constant shifting of allegiances: yesterday's friend was today's enemy, and vice versa. There were alarums and excursions, but in almost every case, once the excitement was over, the army and navy were allowed to rot, with everyone blaming everyone else for Britain's ill state of preparedness. The French assisted in the American War of Independence, but this was a diversion for the real objective: the invasion of Britain itself, in alliance with Spain. News of the outbreak of the French Revolution, 10 years later, according to the author, ''was received with widespread satisfaction in England''. The execution of the French royal family changed all that. The situation in Britain was peculiar during this war: ''For much of it, King George III was incapacitated by madness . . . while the nation possessed an outstanding prime minister, William Pitt the Younger, son of another great war minister, Lord Chatham. While the French made an abortive attempt to capture Ireland, coastal defences in England were being hurriedly built or restored. Thirty million bricks were ordered to build a series of circular Martello towers, many of which survive today. But it was at sea that the French and their Spanish allies were defeated. The proposed invasion of Britain was abandoned. Napoleon turned his troops eastwards, towards Austria and eventually Russia. Then came a series of technical developments. There was Britain's railway system, which could move troops rapidly from one part of the country to another. There was the telegraph, which gave virtually instant communication. And there was a thing called asubmarine. The Royal Navy was not impressed. ''One admiral confidently predicted that 'before very long, we shall hear no more about them' and another described submarines, in a much-quoted phrase, as 'underhand, unfair and damned un-English' ''. Nevertheless, Britain began to build up its own submarine force. It gradually became clear that war with Germany was inevitable. Spies were imagined here, there and everywhere. When war did come, Britain was defended by hundreds-of-thousands of men, quite apart from the British Expeditionary Force sent to Europe. But gradually, as the carnage in France and Belgium grew, more and more drafts were taken from the home front reducing the numbers to laughable proportions. A new proportion was added with air power. The Zeppelin was tried, but was too easily shot down. Conventional aircraft like the big Gotha bombers did, however, cause considerable casualties. That war did, eventually, end, with the Germans metaphorically swearing, hand on heart, that they would never do it again. But of course, they did. Along came World War II. Air raid wardens patrolled the streets after vowing vengeance on anyone breaking the strict blackout precautions. Despite any precautions, nothing much could be done about bombs dropped at random on majorcities. By December 31, 1940, more than 22,000 civilians had been killed. The next year saw another 20,000 dead. But there was never any thought of capitulation. Retaliation was the order of the day. Hitler was forced to give up his elaborate plans for invasion.Instead, it was his own country that was invaded and occupied. A good book this, clearly written although sometimes cluttered with detail. From a practical point of view it would far better have been split into two volumes. A 600-page paperback is an unwieldy animal.