The new Elizabeth line (train) has opened up possibilities with of living out in the countryside and being able to get into central london quickly.
From somewhere like Maidenhead (west of london near Slough mentioned above) you would have a lot of good cycling on your doorstep, an easy commute to central london and it's also not far from Heathrow if that's a consideration.

EDIT: You'd also have plenty of options for open water swimming and there's an evening sprint triathlon series during the summer at dorney lake: https://www.dorneytriathlon.com/


Richmond Park is the best more central option which has a low traffic (outside of rush hour) rolling 6.5mile loop and can feel like the countryside early morning or evening. Geographically you're closer to the city but train commute might not be much quicker - though you could also cycle to/from the office in a similar amount of time.

Daylight should be a consideration though. Outside of the summer months, there might not be much of it outside of work hours - and weather can be more of a factor.. in which case turbo trainer or spin studios.