The Amber grading warns of adverse health effects for the general public, not just the most vulnerable to extreme heat.
The blaze in Chelmsford, Essex, started in a garden then spread to others, destroying sheds and affecting 15 homes.
Heatwave sweeping southern Europe heads to the north of the continent, with authorities declaring national emergencies.
Conservative MP Sir John Hayes said the UK was 'cowardly' for being frightened of the life-threatening heat coming on Monday and Tuesday.
Warnings of heavy rain and localised flooding in much of south England overnight from Friday until Saturday morning.
After a wet and blustery start to the week, temperatures are set to rise in time for the weekend's celebrations.
The Met Office says the rain forecast for early next week could give way to drier weather by the time of the Jubilee weekend.