Both, usually, and the town pages exist to separate the two.
Every town page is built from the site's own stored daily series back to 1950, and shows the count of hot days by decade and the trend through them rather than a single number. The stripes on the homepage are the same series for the country: one band per year, coloured by how far that year sat from the 1951 to 1980 average.
Three things about those stripes are honesty decisions rather than design ones. A year only counts when every town covers it in full. The baseline is fixed rather than chosen to suit the picture. And the colour scale is symmetric, so a warm year and a cold year of the same size get the same strength of colour.
Find your nearest town from the postcode box on the heat data page. If the trend where you are looks different from what you expected, say so here: the towns are twenty two places, not a map, and there will be parts of the country the series does not describe well.