Looking at the 2 references above it looks as though there are a few issues associated with these lists
They show different date records for the same date in some instances
One of them is several years out of date
One of them does not show locations
I wondered how difficult is would be to generate an up to date list with all locations present and with all the differences resolved
And the answer is that it is difficult
The primary source of data on the internet on which all the lists seem to be based seems to be this research by Webb and Meaden from September 2000
https://rmets.onlinelibrary.wiley.com/doi/pdf/10.1002/j.1477-8696.2000.tb04084.x
This itself is an update of previous work in 1984 and 1997 (not found on the internet) which seems to be a painstaking analysis of many disparate sources of digital and paper based data going back to the 19th century. Without a huge amount of endeavour these results cannot be checked easily and IMO should be taken at face value by and large
Other sources of useful information not mentioned in this thread so far are the daily Met Office Weather Logs dating back to the 19th century which give extremes for every day and much other synoptic information
https://digital.nmla.metoffice.gov.uk/collection_86058de1-8d55-4bc5-8305-5698d0bd7e13/
The Met Office monthly reports from 2000 available by Googling (e.g. Met Office September 2002)
and the British Isles Weather Diary from 2000 e.g.
http://www.met.rdg.ac.uk/~brugge/diary2008.html
The Met Office also list the Date records by month for the UK (tab 2)
https://www.metoffice.gov.uk/public/weather/climate-extremes/#?tab=climateExtremes?WT.mc_id=Twitter_Weatherdesk_Enquiries
Having noted the contents of these reports and references I have attempted to produce a consolidated up-to-date list showing all locations in full
I have marked a small number in red where the date record is at variance with the above references although I suspect these may be due to very local weather conditions in that area (e.g. Foehn Effect) as they are included in the Webb and Meaden Analysis
The only genuine issue is the record for 29th March which the Met Office gives as Mepal (Cambs) at 25.6C in 1968 - which the Met Office also cite as the March overall record for the UK but no one else even mentions
Here is my own data FWIW
https://1drv.ms/x/s!Albek-Z8QJdeg0RxMFkF1pYjue2v
Well done Martin. I compiled my own list of maximum temperatures a few years back which I keep updated. I have a few differences in my table to yours which I have listed below.
1 January - my understanding is that the official record is 15.6C set in Bude in 1916. I got this from a Met Office Facebook post which I don't think exists any longer but it is quoted in this article from the Independent https://www.independent.co.uk/news/uk/uk-weather-warmest-new-years-day-in-99-years-while-daring-swimmers-take-dips-in-the-sea-9952879.html
22 February 2019 - 18.6C Colwyn Bay from the Met Office Daily Weather Summary
14 April - I don't think this Southampton reading is correct as I am not aware there were any official weather stations in Southampton in 2007. I think this figure is from the Southampton University Oceanography Centre which is not an official station as far as I am aware, albeit it does have a station reference number. So not entirely sure about this one.
22 June - the Torro figure for this date from 2017 is clearly wrong. However, the original figure shown by the Torro article in Weather magazine in 2000 has Camden Square at 34.3C in 1941 ahead of the Tottenham figure.
5 August - the maximum shown in the monthly weather summary (on the Met Office Website) for 2003 is 35.0C at Jersey Rocquaine
15 October - 22.9C Gogerddan 2017 (per daily weather summary)
29 October - 19.8C Lacock 2014 (per DWS)
9 November - the Thornes Park reading should be 18.6C per DWS
10 November - 20.0C 2015 Tregarth (DWS)
28 November - the Abergwyngregyn figure is actually 18.0C if you look at the Torro site carefully not 17.0C
6 December - 16.5C 2007 Longtown (Met Office monthly summary)
16 December - The forum at COL Weather gives 17.4C at Llanfairfechan on the 16th in 2015. Not sure why this is not being recognised when the reading on 7th in the same month is recognised. Presumably there is a reason.
26 December - 16.1C Banff 2011 (Met Office monthly summary)