I’m not sure I find this convincing, tbh:
Surely if UHI is the key factor, they should have compared Heathrow with St James’s Park, right at the heart of London, and not another suburban station like Kew Gardens.
Also, my understanding is that the main concern about Heathrow is not so much the average monthly means but rather maximum temperatures in the summer, particularly during heatwaves such as in the recent summers, where Heathrow appears to “overperform” compared to other nearby stations. But the article evades this but instead focuses on average monthly means to prove an (all too easy) point that Heathrow and Kew indeed have very similar means year round.
Even during this summer (2021), despite the absence of extreme heat so far, Heathrow has regularly and consistently recorded higher maxes than either SJP and Northolt, and if the rainfall stats are to be believed, has also mysteriously largely escaped the downpours which have plagued much of the southeast.