In the UK. They are roughly 1.5mm long, appear to crawl much more than fly, and seem to live in the soil of indoor potted plants. They are fast and tiny, so apologies for very shallow DOF. The first is on an aloe, the second is on a pot. They really like the soil around some capsicum plants.
Those look to me like fungus gnats. If you're looking to get rid of them, I've heard that putting a layer of rice hulls on top of the soil keeps them from reproducing. From what little I've read my identification doesn't go much deeper than superfamily, taxonomically, but it's a least a common name to work from.