Dave Cornwell wrote:

Peter Gibbs siad this morning that "Yes, there is a heatwave on the
way". Well to me it looks like Wednesday could be quite hot but surely
it breaks down t he next day before we return to unsettled and average
temperatures. Is that what it's come to, one day is a heat wave in a
modern summer. I am already getting people telling me there is a heat
wave on the way and having to lower their expectations.
Dave

It used to be that a "heatwave" was a period of very high (for UK) temperatures,
but now it just means a period of above average temperatures - which is why
they were able to say there would be many more heatwaves in the future without
actually lying.