Here are a few more. I don't quite agree with your figure for 2019. I have it as the 28th June when 30.0C was reached at Achnagart.
The last time we failed to reach 30C in June was in 2014 (we only just made it in 2015). So if we fail to make it this month then we are looking at 18 July in 2014 as the latest first date to reach 30C in recent years.
The Met Office reminded us yesterday that 13 June is the only date in the three summer months that has never recorded a 30C.
2014 18 July (Gravesend 32.3C)
2015 30 June (Heathrow 32.5C - this is the one that notoriously occurred just before 0900 on 1 July but counted as the maximum for 30 Jun)
2016 18 June (Kew Gardens, St James Park 30.0C)
2017 17 June (Teddington Bushy Park 30.2C)
2018 25 June (Hampton Water Works 30.1C)
2019 28 June (Achnagart 30.0C)
2020 24 June (Heathrow 32.6C)