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When the British aren't talking about their gardens and the rain, they are discussing a new coin - the round, thick, heavy, silver-gold but small pound. Tourists, beware. To take a taxi or buy a newspaper here you could now find yourself sorting through eight coins: the half-penny, the penny, the two-penny, the five-penny, the 10-penny, the seven-sided 20 pence piece (small), the seven-sided 50 pence piece (large), and now the stout, milled-edged pound, which is actually smaller than coins worth much less.

The Royal Mint has circulated about 200 million of the new ''round pounds'' to cope with inflation (the pound is worth 30 percent of its value in 1971), and to aid public transport and vending machine operators.

Immediate reaction has been a mixture of curiosity and hostility: ''Too small ,'' said a man at a railroad station. ''Like toy money,'' said another.

A clergyman complained that it was too wide to fit through the slots of collection boxes. Conservative member of Parliament Michael Neubert of Romford said it was confusing to have a high-value coin so small: ''This pseudo-sovereign is a very unsatisfactory little coin,'' he remarked.

The last word went to Mary Marsden from Shropshire. She wrote to a newspaper that all her prejudice about the shape, weight, and color of the coins ''could be overcome by the possession of a sufficient quantity of them.''