I reckon that a temperature of 26.7°C or 80°F is the threshold for a summer day in Central England. If you set the threshold any higher the number of occurrences drops off quite a bit. I analysed the data back to 1878 when the daily series started pushing out both maximum and minimum temperatures, and as you can see from the chart below there has been a 43.1% increase in the annual number of days with a maxima equal or higher than 26.7°C since 1878.
The table below shows the results from three different thresholds (25, 26.7 & 28°C). The table is sorted and ranked on the 26.7°C column, and the year 1995 with 24 days with maximum temperatures of 80°F or higher tops the results, quickly followed by 1976 the year with the great summer.
And here are the results grouped by decade:
Of course some years are worst than others, 2012 had no days in Central England with a maxima above 80°F, so did 2012, and the very bad summer of 1954 (not in this list) only had one day with a temperature above 25.0°C!
Finally, here is a chart of the number of days with a slightly lower threshold of 25.0°C, which as you can see from the chart shows an even greater 59% increase in the annual number of such days since 1878.
In conclusion the CET is showing a strong increase in warm or very warm days in the series back to 1878, the increase is of course erratic as you would expect, but the trend over the last 138 years is upwards, and that means whether we like it or there will be an increasing number of warm or very warms days in Central England in the years to come.