It seems that the really hot days - at least recently - usually happen ahead of a trough or front to the West, rather than when we have a stagnant hot air mass sitting over the country in high pressure. i.e. a plume. Doesn't mean the trough ever reaches the SE, it may retrogress as is forecast this time or it may move across but weakened as happened a week ago. But this pattern gave us the record on 10 August 2003, and the record last year, the July record in 2015 as well as last week's 1 day effort.
August 1990 was more of a stagnant airmass as were the peaks of temperature at around 35C in 1976, 1995 and 2018. Can't remember what the peak in July 2006 looked like.
Classic warm conveyor I think, with some katabatic heating over the downs and enough Southerly flow and mixing to prevent a temperature inversion.
Brockley, South East London 30m asl