Ben cambs, east anglia and the southeast did have a level 2 heat health warning last summer around early-mid July. Think around the 8-12th. Milhouse, not sure I would say 9 days of 26c would be too impressive in one month. I'm sure june and aug 09 got thereabouts and june/July 10 also in essex, an avg temp of 23c means 26c is only 3 above. My definition of a heatwave is the met measure of 31c / 16c for the southeast. Anything between 25-30c is very warm for me but i probably wouldn't regard it as a heatwave. With your measure isolated frost, a heatwave would be 28c max at day.