I think there’s only one official sunrise and sunset time for a given location. Sunrise is the time when the upper edge of the sun appears over the horizon and sunset is when the upper edge of the sun disappears below the horizon.
There’s a variety of different definitions for “Twilight”, which is when the sun isn’t visible, but it’s still light. The sunrise-sunset website says:
“WHAT EXACTLY ARE THE SUNRISE AND SUNSET?
It’s important to note that both sunrise and sunset are ‘instants’.
The time range during which the day becomes night or vice versa is called twilight. We can distinguish between the morning twilight, that happens between dawn and sunrise and the evening twilight, that happens between sunset and dusk each day. Duration of twilight actually depends on our position on Earth and date of year. For example at Arctic and Antarctic latitudes, on winter, night never gets completely dark.”
For London the sunrise-sunset API gives these numbers for today:
and the Dopmogeek website gives these numbers:
They both agree that the day will be a fraction under 8 hours long, its just that the Domogeek times are an hour later (and therefore wrong by an hour) compared to those from sunrise-sunset.org
Fortunately the OP only needed an approximate time to water his plants, and either set of times would probably have been okay, but I flagged it up because the inaccuracy of the Domogeek data in relation to local time might catch people out in some more critical situations.
I suppose that for watering plants it is actually the sunrise/set times that are needed, but for most timing applications people will probably be thinking about when lights need to be switched on and off - in which case one of the various flavours of twilight will be more useful to them, so your sunrise-sunset API is probably the best choice anyway.
Time to go and make a coffee and watch the sunrise