Interesting news article from the BBC, pointing out the very high temperatures that Heathrow Airport often records during heat waves. Some recent examples of heat measurements here are:

35.0C on 26 July this year

34.5C on 21 June 2017

34.5C on 13 September 2016

36.7C on 1 July 2015 (the July record of course)

Also worth pointing out that the Heathrow weather station has been criticised and challenged on a few occasions, as a result of the very high temperatures it records. 1 July 2015 and 13 September 2016 are just two examples of this.

https://www.bbc.com/news/uk-44980493?ns_campaign=bbcnews&ocid=socialflow_facebook&ns_mchannel=social&ns_source=facebook

With the extensive amount of tarmac at Heathrow, I'd expect it to be a small heat island all of its own, as hinted at in the original link. So to me the surprise is that Heathrow is regarded by the MetO as a reliable place for setting record high temperatures. Nearby Kew Gardens, which also does 24-hr recording, would seem to be more typical - though there's an argument to say that heavily built up areas are now such a feature, that such areas are the new normal environment.

A lot of tarmac yes, but also alot of grassland between the taxiway, the airport actually has a "silage" operation in the spring, and a fair bit of green space and waterways, with two reservoirs just off the western top of the runways, and a surprisingly large amount of arable farm land.

Not sure where you got the 34.5c reading for 13th September 2016 from? It reached 32.1c that day according to the Heathrow records.

I posted this same article on NW - it is an interesting conversation. My opinion is that Heathrow has a number of factors in its favour - it is in the SE of England (hottest area of the country), it is within Greater London so benefits from the UHI effect, it is far enough away from the south coast, the east coast and the Thames Estuary so is an inland location. 

As the article states, however, there is very little difference between the Heathrow records and Kew, which is a centrally located weather station in London, and is actually very similar to St James's Park in central London and Northolt in NW London just up the road from Heathrow. 

It's not actually all that surprising it often tops the table for hottest temperatures. it's in the warmest part of the country in summer, it's located within the boundaries of the biggest city in the UK by far and is sheltered from cooling sea breezes. 

Looks like I looked at the wrong one. 13 September 2016 saw 34.4C at Gravesend, not Heathrow lol.

The weather station at Heathrow is on the grass but between the runway and the main road, very close to the road.
I think this is it:

Google Maps link

Kew Gardens is showing 32.4C at 16:00 against 32.2C at Heathrow

Kew is about 5 miles away from Heathrow and I know the weather station is well away from roads and buildings in the middle of the Botanic Gardens

Makes me think that any errors due to local effects at the airport must be quite small