What's your ideal number of days depends on the criteria?

Which London station is that? It's not Heathrow.

Daily High ≥25°C: 185 days
Daily High ≥30°C: 35 days
Daily High ≥35°C: 5 days
Daily Low ≤0°C: 0 days
Daily Low ≤-5°C: 0 days
Daily High ≤0°C: 0 days

Daily High ≥25°C: 60 days
Daily High ≥30°C: 10 days
Daily High ≥35°C: 2 days
Daily Low ≤0°C: 180 days
Daily Low ≤-5°C: 120 days
Daily High ≤0°C: 100 days

Daily High ≥25°C: 97 days
Daily High ≥30°C: 66 days
Daily High ≥35°C: 30 days
Daily Low ≤0°C: 217 days
Daily Low ≤-5°C: 195 days
Daily High ≤0°C: 171 days

For Heathrow, the data is as follows:

Highs >25c: 28.6 days
Highs >30c: 4.5 days
Highs >35c: 0.2 days
Lows <0c: 32.6 days
Lows <-5c: 1 day
Ice days: 0.9 days

Has Owen got a house on a hill just outside Paris now?

I see you just edited it!

High >25: 110 days
High >30:  60 days
High >35:  18 days
Low  <0:   153 days
Low <-5:    99 days
High <0:    29 days

Now for Rochester:

High >25:  80 days
High >30:  21 days
High >35:   .6 days, occurs 1 in 3.3 years
Low  <0:   131 days
Low <-5:    69 days
High <0:    47 days

Now added more cities on the table.

≥25: 180
≥30: 30
0 for the rest.

Some basics for Darwin.
24c - only 5 days since 1942 below 24c max temp. ( record low max 21.1c in 1968 )
30c - only 40 days per year below 30c ( since 1998 the figure has dropped to 27 days per year and 16.5 days 1998-2020 )
35c - generally 12.5 days per year above 35c ( since 1998 the average is 21.5 days above 35c and 2019 and 2020 both exceeded 44 days )|
15c - minimums 196 mornings @ 15c or below since 1941. 13 in 1965, 11 in 1942, 11 in 1949, 10 in 1967, 10 in 2011. ) Record 10.4c in 1942