BRITAIN is thirsty - and the government is running short of reasons why a normally ''green and pleasant land'' is being forced to ration. A heat wave has produced the driest English summer since 1727, and the Labour opposition party is putting the blame squarely on recently privatized water companies, saying they are failing customers while reaping huge profits and wasting resources. Water has been a sensitive political issue in Britain ever since the industry was privatized six years ago. In this parched season, it is at the center of a national debate over how much privatization is in the public interest. Environment Secretary John Gummer said Aug. 21 that he would would like to introduce voluntary metering of individual households. Only a small proportion of British households now have water meters. Most users pay for water as a proportion of local taxes, and consumers' groups oppose the introduction of meters, saying companies themselves rather than individuals should be held more accountable for wasting water. Meanwhile, the same country where umbrellas are usually a necessity is gasping for relief. The county of Yorkshire, for example, is suffering its driest summer in 500 years. On Aug. 21 it was officially declared a drought area, with water having to be drawn from a local river. When the decision was made to privatize the nation's 10 main water companies, the government said future shortages were unlikely because the companies would have to take preventive measures. But the companies say 25 to 30 percent of available water is lost through leaking reservoirs and fractured pipes and add that it would be uneconomic to boost significantly spending on repairs. THE Labour Party and consumers' groups say companies are are not assuming enough responsibility. ''It is unfair to accuse ordinary citizens of wasting water while companies supplying it are guilty of the same offense,'' says Sheila McKechnie, director of the Consumers' Association, Britain's most influential consumer pressure group. ''What is more, in some areas, since privatization, the price to the consumer has nearly doubled.'' Mr. Gummer says that it is up to the water companies to set their own targets for spending on infrastructure. He says total investment is already $4.5 billion a year. The water industry was privatized in the face of angry opposition by the Labour Party. Friends of the Earth, a high-profile environmental group, warned that putting such an essential commodity into private ownership ran against the public interest. But Conservatives insisted that like electricity, gas, and North Sea oil, the water industry would be more efficient outside the public sector. They did not reckon with the weather. The last serious drought year here was 1976, but its severity was nothing like what is being experienced now. Reservoirs in five of the 10 big water companies are running less than half full as evaporation takes its toll.