The hottest days of the year in the UK going back to 1875. The majority of the list was taken from the once-existing UK annual maximum temperature list from TORRO although updated and changed over the last few years by me. I will be happy to correct and update any years based on any new data if it turns up!
19th Century
1875
30.1°C Hillington 16/08
1876
35.0°C Cambridge 14/08
1877
29.7°C Stratfield Turgis 18/06
1878
33.9°C Llandudno 27/06
1879
26.8°C Hillington 29/07
1880
30.7°C Loughborough 04/09
1881
35.0°C Camden Square 15/07
1882
29.2°C Portsmouth 06/08
1883
28.7°C Norwood & Tunbridge Wells 29/06
1884
34.1°C Hillington 11/08
1885
33.3°C Southampton 26/07
1886
31.9°C Southampton 04/07
1887
31.7°C Cambridge 03/07
1888
31.1°C Cambridge 25/06
1889
30.3°C Regents Park 01/08
1890
28.3°C Llandovery 06/09
1891
30.0°C Loughborough 10/09
1892
29.4°C Cambridge 03/07
1893
34.7°C Wryde 18/08
1894
30.6°C Weston-super-Mare 01/07
1895
30.6°C Stratfield Turgis 25 & 27/09
1896
31.2°C Norwood 14/07
1897
32.2°C Cambridge 05/08
1898
33.4°C Norwood 07/09
1899
32.4°C Cambridge 25/08
20th Century
1900
35.1°C Cambridge 20/07
1901
33.3°C Bawtry & Colly Weston 19/07 & Newton Rigg 20/07
1902
31.9°C Hereford 28/06
1903
31.1°C Wryde 10/07
1904
32.2°C Margate 04/08
1905
30.0°C Bawtry 14/07
1906
35.6°C Bawtry 01/09
1907
30.0°C Lairg 16/07
1908
32.8°C Dumfries 02/07
1909
33.3°C Epsom 15/08
1910
28.9°C Maidenhead 20/06
1911
36.7°C Raunds & Canterbury 09/08
1912
32.8°C Tottenham 12/07
1913
29.4°C New Malden 16/06
1914
32.2°C Woking, New Malden & Wisley 01/07
1915
32.2°C Cromer & Norwich 08/06
1916
30.0°C Woking 30/07 & Salisbury 31/07
1917
33.9°C Reading & Little Massingham 17/06
1918
33.9°C Canterbury 22/08
1919
32.2°C Raunds 11/09
1920
28.9°C Cambridge, Botanical Garden 25/05
1921
34.4°C Woking & Halstead 11/07
1922
32.8°C Camden Square 22/05
1923
35.6°C Camden Square 13/07
1924
32.2°C Camden Square 12/07
1925
33.3°C Hunstanton 22/07
1926
32.2°C Camden Square 19/09
1927
29.4°C Camden Square 16/06
1928
32.8°C Camden Square & Newport (IOW) 15/07
1929
32.2°C Camden Square & Margate 31/08 & Newport (IOW) 05/09
1930
34.4°C Camden Square 29/08
1931
28.3°C Bromley 14/06
1932
36.1°C Halstead (Essex), Tottenham, Regents Park, Enfield (all London) 19/08
1933
34.4°C Margate & Cambridge 27/07
1934
33.3°C Attenborough 11/07
1935
33.3°C Attenborough 13/07
1936
31.7°C Camden Square 20 & 21/06
1937
33.3°C Canterbury & Tunbridge Wells 07/08
1938
30.6°C Camden Square & Reading 01/08
1939
32.2°C Camden Square 07/06
1940
32.8°C Cranwell 09/06
1941
34.3°C Camden Square 22/06
1942
33.9°C Sprowston 27/08
1943
33.9°C Worcester & Croydon 31/07
1944
32.8°C Tunbridge Wells, Horsham, London 29/05 & Long Sutton 30/05
1945
32.2°C Norwich & Whitstable 15/07
1946
30.6°C London 02/07, Finningley 12/07 & Greenwich & Maldon 24/07
1947
34.4°C Waddington & Kensington Palace 03/06
1948
35.0°C Milford 28/07
1949
33.3°C Worcester 12/07
1950
33.3°C Camden Square 06/06
1951
30.0°C Southend-on-Sea 28/07
1952
33.3°C Camden Square, Heathrow & Southampton 01/07
1953
33.9°C Camden Square 12/08
1954
30.6°C Camden Square 01/09
1955
32.2°C Chivenor 23/08
1956
30.0°C Camden Square 29/06
1957
35.6°C Camden Square 29/06
1958
29.4°C Paisley 04/07 & 05/07 & Leicester 08/07
1959
34.4°C Cromer, Boxworth & St James Park 05/07
1960
30.6°C Wyton 18/06
1961
33.9°C Regents Park, Camden Square & Gillingham 01/07
1962
27.8°C Writtle 03/09
1963
28.9°C Littlehampton 28/07, Ceinws 29/07 & Wisley & Gordon Castle 30/07
1964
32.8°C Cromer 26/08
1965
29.4°C Camden Square 14/05
1966
28.9°C Southampton 09/06, Perth & Crossmyloof 21/07 & Camden Square 20/08
1967
30.2°C Watnall 17/07
1968
33.3°C Camden Square 01/07
1969
32.8°C Letchworth 16/07
1970
32.2°C Aldenham & Stratford-upon-Avon 07/07
1971
30.3°C Rugby 11/07
1972
29.4°C Perth 20/07
1973
32.1°C Southampton 14/08
1974
28.0°C Southampton 15/06
1975
34.2°C Heathrow & Stanstead Abbotts 08/08
1976
35.9°C Cheltenham 03/07
1977
30.0°C Paisley & Glenlee (D. & G.) 07/07 & Onich 11/07
1978
28.8°C Poolewe 04/06
1979
30.4°C Benson 27/07
1980
29.4°C Cors Fochno 04/06
1981
29.5°C Kew Gardens 05/08
1982
30.7°C Morley St. Botolph 03/08
1983
33.0°C East Bergholt 15/07 & Hampton 16/07
1984
31.7°C Heathrow 08/07
1985
30.0°C Southampton 25/07
1986
32.0°C Rustington 28/06
1987
30.2°C Liphook 21/08
1988
30.2°C Cheltenham 07/08
1989
34.4°C Mickleham 22/07
1990
37.1°C Cheltenham 03/08
1991
32.1°C Cromer 11/07
1992
30.3°C Malvern 29/06
1993
29.7°C East Bergholt 04/07
1994
33.4°C Morley St. Botolph 12/07
1995
35.2°C Boxworth 01/08
1996
33.2°C Rickmansworth 22/07
1997
33.1°C Wellesbourne 10/08
1998
32.2°C Gravesend 10/08
1999
32.7°C Heathrow 01/08 & Honington 02/08
21st Century
2000
32.6°C Coltishall 19/06
2001
32.2°C Northolt 26/06
2002
32.6°C Northolt 29/07
2003
38.5°C Brogdale 10/08
2004
31.5°C Northolt 08/08
2005
32.6°C Heathrow 19/06
2006
36.5°C Wisley 19/07
2007
30.1°C Terrington St. Clement & Wisbech 05/08
2008
30.2°C Cambridge 28/07
2009
31.8°C Wisley 30/06
2010
32.7°C Brogdale 09/07
2011
33.1°C Gravesend 27/06
2012
32.4°C Cavendish 18/08
2013
34.1°C Heathrow 01/08
2014
32.3°C Swanscombe Marsh 18/07
2015
36.7°C Heathrow 01/07
2016
34.4°C Gravesend 13/09
2017
34.5°C Heathrow 21/06
2018
35.6°C Felsham 27/07
2019
38.7°C Cambridge Botanical Gardens 25/07
2020
37.8°C Heathrow 31/07
2021
32.2°C Heathrow 20/07
2022
40.3°C Coningsby 19/07
2023
33.5°C Brogdale 10/09