What insect is this?
Found in the UK, approximately 10mm-15mm in length.
That insect looks quite like a Mayfly, it's an ancient insect that belongs to the order Ephemeroptera. I'll go a step further and guess that species, that one there is most probably a Green drake, they aren't harmful at all as in they don't sting or bite but if a lot of dead mayflies pile up they can cause respiratory inflamations, asthma, hay fever that sort of stuff.
They're a vital part of the pond ecosystem. Here's a picture of a Green drake and it's quite similar to your picture. Credit to Missoulian Angler I think some of the major reasons why it is a Mayfly are a s follows:-