A 'hot' summer's day would be anything above 25C, with 30C classed as unusually hot. There have been days when the temperature went up to 35C and even some at over 38C (100F) but that's exceptionally hot, we're talking maybe once a decade at most.
London is a considerable heat island, so during long periods of hot weather the heat accumulates and feels considerably more stifling than the same air temperature in the countryside.
There are no useful averages in UK, because the weather is much more variable than you are used to elsewhere. I've seen 4 kinds of weather in one morning, and that is not unusual.
The hottest summer days in London can be over 30C and humid (that's 86F to you)
The whether here is famously unpredictable, but London gets pretty hot at times, often 30 degrees (Celsius) or even slightly above. Also, the amount of concrete and sheer number of people in London tend to make hot days feel even warmer than they actually are.
Normally up to 25 to 30 degrees centigrade, but it can get hotter during heat waves
www.metoffice.gov.uk
