britain 's heatwave sends ice creams sales soaring - with some vendors seeing a 400 per cent rise in sales .
temperatures forecast to hit 21c tomorrow , with the country set to be warmer than ibiza , athens and barcelona .
but the balmy conditions could be coming to an end , with clouds , wind and a spot of rain forecast for thursday .
showers predicted to hit london on sunday as tens of thousands of runners take part in the london marathon .