I think they're black garden ants and are a mixture of males and females that fly away from the nest in search of a mate from another nest. The males will die within a couple of days and the females will establish their own colonies.
In amongst all the black winged ants were a few smaller red ants (you can see one in the picture below) that seemed to be overseeing the launch site.
Interestingly we heard (but couldn't see) a green woodpecker in the garden earlier in the day. Wonder if it knew the ants were about to fly!
The Royal Society of Biology run a flying ant survey each year, trying to track when the ants fly around the country. We've reported our sighting and you can find out more at https://www.rsb.org.uk/get-involved/biologyweek/flying-ant-survey/.