London — In the Dorset countryside 100 miles southwest of here, visitors can see donkeys nodding in the sun - or, to be more accurate, ''nodding donkeys'' in the sun. These beasts are oil pumps quietly extracting 4,000 to 5,000 barrels of crude oil a day from strata 4,000 and 6,000 feet down.
Britons can expect to see more such mechanical creatures at work in the future. There's something of an onshore oil boom on this island.
Onshore oil, notes Roger Bexon, deputy chairman of British Petroleum PLC, ''is minute compared to offshore oil production and the totality of reserves offshore. It is a curiosity rather than anything of importance.''
Nonetheless, many oil companies are racing to find more oil under solid ground. Compared with the billion dollars or so cost of building an offshore production platform, the money needed to sink an onshore well is tiny - say $500 ,000 to $1.5 million. This permits small oil companies as well as such giants as BP to scramble for onshore oil prospects.
Moreover, the ratio of successful to dry wells has been ''quite high so far, '' notes Mr. Bexon. This further reduces the risk factor.
Last year the Department of Energy awarded a record 63 licenses for exploration or production, mostly in England, several in Scotland. ''There is a lot of activity,'' commented a department official.
The Wytch Farm oil field in Dorset, with perhaps 200 million barrels of reserves, is bigger than all other onshore fields found so far combined. It is about the size of one of the smaller pools of oil offshore.
BP is operator of the field, owning a half share. The other half was owned by British Gas Corporation, a nationalized company. It is under orders from the pro-free enterprise government of Prime Minister Margaret Thatcher to sell its oil properties to private industry.
British Gas was reluctant to unload Wytch farm, resulting in what Mr. Bexon calls ''a minor but fairly intense political event.'' However, after a delay of more than two years, the sale to a consortium of five independent oil companies was completed in May. The BP executive now expects development of the field to go ahead ''more rapidly,'' with production expected to reach 20,000 barrels daily.
The presence of oil has long been known in Britain. The Romans used pitch, found in Shropshire springs, for waterproofing. Then in the 17th and 18th century, Shropshire landowners made a useful income by setting afire oil or gas escaping from what appeared to be ordinary wells, showing this to travelers. They also used the seepages as lamp oil or medicine.
Some use was made of oil seeps found in coal mines. And in Scotland oil shale provided most of the oil the nation needed before cars became the popular means of transport. Some 265,000 tons of shale oil were produced in 1913. Huge mounds of shale talings still dot the Scottish lowlands.
Just before World War II BP drilled the nation's first successful oil wells at Eakring - a strategic find outside the range of U-boats. Peak production for that period reached 115,000 tons per year in 1943.
Production last year amounted to 315,000 tons, produced mostly from small fields with such colorful names as Humbly Grove, Palmers Wood, and Bottoms Copse. That compares with 111 million tons from offshore. In the current onshore oil rush, the biggest problem for the oil companies is dealing with local environmental concerns.
''This is a fairly crowded island,'' remarked Mr. Bexon. ''So developments don't take place so easily as in the southwest United States.''
Oil companies must also often face opposition from landowners. Since 1934 all oil under United Kingdom soil belongs to the crown - the government. So, for a landowner, the discovery of oil can be nothing but a nuisance, aside from possibly renting a small surface area for the drilling or production machinery. He or she receive no royalties.
BP and its partners have had to deal carefully with the environmental questions in Dorset, a relatively unpopulated area considered a beauty spot and the site of Thomas Hardy's novels. Mr. Bexon describes Dorset environmental rules as ''very strict.''
Nodding donkeys are often hidden behind hedgerows. Oil tanks are camouflaged behind walls or fir trees.
''There must be as little disturbance to the countryside as possible,'' said Bexon. Special care had to be taken not to disturb the Dartford Warbler, a rare bird living in the area. ''It looks like a sparrow,'' noted Mr. Bexon. ''Rather disappointing.''
With the new interest in onshore oil, the Department of Energy plans to issue new environmental guidelines for local authorities by the end of this year. If an oil company is unhappy with a local ruling, it will continue to be able to appeal to the secretary of state for energy and in some cases courts of law.
Moreover, the Department of Energy announced April 4 new arrangements providing for three types of onshore licenses, dealing with exploration, appraisal, and development. The exploration license will be for six years, allowing an oil company to search and drill for petroleum once he gets necessary permission from landowners and occupiers and planning authorities. Prolonged testing of any find will require an appraisal license good for five years. And the development license will be for a 20-year term, extendable at the discretion of the secretary of state.
So far, about one-quarter of Britain's land mass is covered by licenses of one kind or another. Mr. Bexon says it is unlikely a major pool of onshore oil will be discovered. But such finds, though small, continue to multiply and generally prove highly profitable.