The UK's spell of toasting under clear skies is coming to an end, according to forecasters, with cooler but still pleasant weather predicted for most of the country over the diamond jubilee weekend.

The shift will have a dramatic prelude in some areas, with the first severe weather warning for weeks appearing amid the yellow suns on the Meteorological Office website on Monday.

Thunderstorms and torrential rain are expected to hit parts of the south-east, including London, from 3pm. A yellow "be aware" warning – the third most serious – has been posted for the Thames estuary and extending into Cambridgeshire, Essex and Kent. The Met Office says that summer convectional storms, when very hot air rises and cools suddenly, will threaten "torrential downpours" until 11.30pm with the risk of flash flooding, especially in urban areas with little open ground to absorb the rainfall.

The glorious run of scorching weather, with temperatures reaching 27°C (80.6°F) on Sunday and expected to come close to that before nightfall, has been marred by a series of drownings. Police in Lancashire have found a body in a reservoir at Yarrow Valley country park near Chorley, where a 15-year-old boy was reported in difficulties in the water on Sunday afternoon.

The tragedy follows the death last week of another 15-year-old, Hussain Mohammed, in the river Thames at Oxford. Described by his family as a "happy, cheeky and popular teenager", he had jumped from a bridge to cool off in the river.

Meanwhile, police in Glasgow named a man who died near a waterfall in Linn Park as 23-year-old Nicholas Smith. In Ballykelly near Monasterevin, a 22-year-old drowned in a disused quarry and the body of a 21-year-old man was recovered by police in Milton Keynes from Caldecotte lake, where he had gone swimming.

The deaths prompted renewed warnings about the dangers of swimming in inland waters that remain extremely cold, even in very warm weather. Water companies issued reminders that reservoirs are particularly dangerous, with water abstraction causing strong currents.

Sussex police are expected on Monday to name a man who died off West Wittering beach while trying to help two children who got into difficulties playing with a rubber ring. Officers described the 25-year-old, who was not related to the children, as a Good Samaritan, and crowds who were on the beach on Saturday praised the quick response of emergency services and the air ambulance.

The warm weather is expected to cling on for longest in north-east England, where the chances of sunny parties and melting cupcakes look strongest over the jubilee weekend. Forecaster Sally Webb of MeteoGroup said temperatures were likely to drop overall from peaks of 27°C on Monday to 17°C by the end of the week, still at the top end of averages for May.

The weekend's hottest temperatures were Kinlochewe in the Highlands, which reached 27.9°C on Saturday, and Wisley in Surrey, home of the Royal Horticultural Society's gardens, which touched 27.4°C on Sunday. Porthmadog in north Wales was not far behind with 26.9°C.

North-east Scotland will see the biggest drop in temperatures, as tenacious arctic airflows try to move south, but the longer term forecast is for a gradual improvement. The Met Office's maximum 30-day prediction says that conditions in June will be changeable, although temperatures will be "near or above the seasonal average, but perhaps cooler in the south".

The sunshine has been handy for owners of photovoltaic solar panels, especially the West Mills co-operative at Watchfield, Oxfordshire, which has 20,260 panels on 30 acres of land. Farmer Adam Twine said: "Lots of sunshine is just what we like here – we've had a great few weeks."

Transport has also been affected by the heat, including the London underground. Severe delays affected the Circle, District and Hammersmith & City lines,.

New gravel laid on top of melting blacktop tarmac in Gloucestershire was also blamed for creating "treacle-like" conditions by drivers. A 43-year-old teacher from Chipping Norton, Julie Stoddart-Cook, died in one of three accidents on one of the roads affected. Kath Haworth, of Gloucestershire Highways, said: "We carried out surface dressing on the road some weeks ago but, following the recent hot spell, the surface began to 'sweat'. So at the end of last week, we dusted the surface with fine grit and we also put out 20mph speed signs to warn drivers. We are working with the police on their investigation, so it would be inappropriate to speculate about the cause at this time."