London recorded 36.4C on 26 June and 35C again on 9 July, both before the 10 August deadline, per Met Office readings reported by the Evening Standard and Time Out.
The 35C market resolves on official Met Office daily maximums from London stations (Heathrow, St James's Park, Kew Gardens, Northolt); the highest verified reading at any of these counts, provisional figures announced on the day stand unless corrected within 72 hours, and media-reported unofficial readings do not count.
London passed 35C in three of the ten summers before this one. We looked at when its biggest heat spikes came. They are short and sharp, and they usually land in late July or August. A hot June has not meant the peak of the summer was over.