London — Most of Britain's 20 million households, it seems, are out in the garden these days as spring gently washes over the grays and browns of winter with sheets of color.
Everywhere islands are emerging from drenching April rains dressed in the yellows of primrose and daffodil, the pink and purple of tiny aubrietia bursting cloud-like from borders and cracks in walls, the purple of pansies, red of tulips, white of alyssum.
Families drive to their local garden centers to fill back seats and trunks with potions to kill weeds, with seedlings and potted plants, with fuchsias, geraniums, and trailing lobelias for hanging baskets on porches and walls.
Certainly this country takes its gardening seriously. The Times of London lists gardens open for public viewing around the country. The Times says solemnly:
''A Suffolk field containing one of Britain's rarest wildflowers, fritillary, is open to the public today. It is opened on only one day each year. . . .''
Already it is clear that one of the biggest events of next year will be the first international garden festival ever staged in Britain, set to be opened by the Queen in April 1984 on 250 acres of reclaimed dockyards in Liverpool. More than 3 million are expected to pore over garden exhibits from Italy, Japan, and many other countries.
Gardens are big business as well. The House of Commons itself has taken note of a head-on battle for the right to mow Britain's lawns. One of the biggest makers of lawn mowers has run television advertisements claiming that its rival's machine - which hovers on a cushion of air as it cuts - is inefficient.
The company has spent a fortune on trumpeting that its own machine is a ''lot less bovver than a Hover.'' Hover, upset by such aggressive counteradvertising, cries foul play, and headlines sprout.