Olympic silver medallist Michael Jamieson was also at the British Swimming Heroes event at the Gurnell leisure centre in Ealing.
He told Emma Walden that it was an exciting time to be involved in the sport.
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The high of 35C was recorded on a Bakerloo line train between Piccadilly Circus and Oxford Circus at 2pm.
Weather forecasters say temperatures could reach 35C, beating the previous record of 34.6C in the scorching summer of 1976.
After years of political manoeuvring over the future expansion of Heathrow Airport, there has been an important development this lunchtime.