According to my weather book, 30C occurred on the 16th of April 1949 in
London.

33C occurred on the 22nd May 1922, again in London.

The highest ever June temperature (and again for London) was 35.5C and
occurred on the 29th June 1957.

The exact details of where in London these temperatures were recorded is not
stated.
________________
Nick G
Otter Valley, Devon
70 m amsl
http://www.ottervalley.co.uk