Swallows have much longer tails with a very deep fork, and they have white bellies. Swifts are dusky blackish all over (apart from a tiny pale chin patch. The low swooping behaviour is more typical of Swallows, Swifts spend more time flying high :)
ETA - a Swift:
A Swallow:
It's also possible you're seeing House Martins (shiny black with white underside and rump), and Sand Martins (mid-brown with white underside, brown breast-band and no white rump).