Are you sure you’re thinking of last year and not the year before? 2024 was a relatively cool summer but 2025 was the hottest summer on record in the UK (probably set to be broken by this year).
As for other recent years, 2023 had the warmest mean June temperature ever, a record which was subsequently broken again in 2025 and 2026. 2022 recorded the hottest and 4th hottest days ever.
2024 is looking like the outlier, not the heatwaves.