UA 5209 then BA 9603 at Houston
Tucson → Houston → London
Timetable, Houston time
12:55UA 5209 from Tucson at the gate
17:10BA 9603 to London leaves the gate
4 h 15 apart on the timetable
The last days
UA 5209 and BA 9603 weren’t both flown 15 min to 12 h apart in the last 21 days.
Statistics from past flights, not live flight information. Always check times and your connection with your airline, and don’t base travel or booking decisions on these numbers.