I love how a period of nice weather in the UK now gets called a heatwave, maybe I'm remembering wrongly but in the 90's it was a heatwave unless it was a week of temps closer to 40c
I know what you mean - no idea as to the official side of it but in the 80s and 90s upper 20s, basically anything below 30, wasn't thought of as exceptional - was when the temperatures were topping 30s for several days people started to talk about a heatwave. A few days of maybe hitting 28-29C was just regular summer.
On some models there is a chance of parts of the south east seeing closer to 40 towards the middle of the month... probably won't quite get there but potential is there.