I find these to be quite a stunning insect to look at. They belong to an order of insects called Mecoptera. There are less than a handful of species in Britain, and they can be awkward to tell apart. The above image is that of the female. The males have large bulbous genitalia, which looks kind of similar to the stinger of a scorpion, hence the English name. Although they look like they might sting, these flies do not.
I always find these Scorpionflies a challenge to photographs because they do not hang around for long once they sense you are near, unless you can get them from long-range. I got fairly close to this one which was resting on vegetation by the local river before she flew off.
Taken July 2017.
Please click on an image for a larger more detailed view. Clicking a second time may get you a little closer.