London — British pride swells when talk turns to the sea. Virtually all British people live within a few hours' drive of the coast. The sea is Britain's history, its romance, its defense, and its lifeline. Defended by Drake and Raleigh, enshrined by Shakespeare, Kipling, and Masefield, it forms an incoming highway for 40 percent of the nation's food supplies.
Threaten or challenge Britain at sea, and you awaken the nation - and that is what Denmark has done in the latest clash over European fishing rights.
At this writing, the dispute seemed likely to end up in the European Court of Justice - with both sides saying they would win. But meanwhile it was threatening to escalate far beyond the wishes of either the British or Danish government.
For her part, British Prime Minister Margaret Thatcher, fresh from asserting British sea power and identity in the Falkland Islands, refuses to shrink from Danish fishing boats - especially when her position is supported by eight of the nine other Common Market states and opposed by Denmark alone.
So her spokesmen, while urging Danish fishermen to abide by the rules of the nine, make it clear that British ships and planes will protect mackerel, cod, plaice, and other fish in British waters. A full two-thirds of European fish breed and swim off British coasts.
If necessary, British officials will board Danish ships and order them to port to face British law - to the support of newspapers here, and as far as can be judged, of public opinion.
It faces another proud seafaring nation, whose government, ironically, is as anxious to prevent open clashes as Mrs. Thatcher.
The two countries have been allies almost since the days of the Viking raids on English coasts more than a thousand years ago. But the Danish Conservative Party is a minority government - and in the Parliament in Copenhagen a majority of votes have rallied to the aggrieved and unyielding position of the men on Danish boats.
It is not another ''cod war'' such as the one Britain and Iceland fought in 1975. Then, armed ships on each side confronted each other to assert fishing quotas. Britain eventually lost its rights in Icelandic in-shore waters, and its distant fleet was decimated.
But the current dispute is serious enough. At this writing, Danish boats were heading for the coasts of Scotland, the Shetland and Orkney Islands, and southeast England, determined to fish for cod, mackerel, plaice, and others.
Most Danish boats have indicated they will obey their government's call for restraint and will stay outside the British 12-mile limit. But one in particular , owned and captained by European MP Kent Kirk, intended to fish for sprat off the Northumberland coast to test British law.
Mr. Kirk indicated he would follow the sprat inside the British limit if necessary. One of his aims is to publicize the Danish case. He had invited 23 Danish and British newsmen onto his small, five-man ship, but officials in Esbjerg intervened, and the newsmen had to charter a second ship.
Helping the episode dominate the news here was the new year holiday period, and a shortage of competing stories.
The British case for protecting its fishing grounds goes like this:
Tremendous overfishing by foreign ships has laid waste herring stocks in the North Sea and threatens mackerel grounds to the south of England. British fishermen, for all their heroic image, are in trouble. Their labor costs are high, their equipment old, and other countries are able to sell processed fish back to Britain at low prices.
British eating habits have also changed: Only one-third of fish eaten here is fresh or fresh-frozen; the rest is processed into such foods as fish fingers. Britain may be surrounded by the sea, but fish consumption dropped to just over one-quarter of a pound per person per week by 1980.
When Britain and Denmark joined the Treaty of Rome that created the Common Market, they agreed that all member states could fish in one another's waters - and that a 10-year ''derogation,'' allowing ''breathing space'' controls within the 12-mile limit, should end Dec. 31, 1982.
Late last year all members except Denmark agreed on a Common Fisheries Policy (CFP) to conserve stocks and share catches according to percentages and tonnages for cod, haddock, whiting, plaice, saithe, red fish, and herring. Britain's share was highest (around 36 percent, though fishermen here wanted 45). Denmark's was next highest at 23.
Countries retain exclusive rights within six miles. Between six and 12 miles, limited rights in some areas are granted to nations that have traditionally fished there.
For Denmark that means no mackerel fishing off the southwest of England and no fish for human consumption in the so-called ''Shetland box.'' Denmark badly wants both.
Its boats can still enter the Shetland box for industrial fishing (for varieties used in fish meal and for fertilizer). Industrial fishing is not covered by the CFP.
But because Denmark rejected the CFP, Britain says it cannot fish inside its 12-mile limit at all. A spokesman for the Ministry of Agriculture, Fisheries, and Food said Jan. 4 that Britain was waiting for Denmark to accept the CFP.