Kill them with fire

[Russinating]

Two of these horrors just invaded the office. What on earth is it?! Was about 2 inches long when flying.

Bigger version: https://i.imgur.com/XsOvZag.png

 

[Russinating]

Dont kill it with fire. The security guard has asked you before not to start fires in your office!

Very good, sir.

I googled: biggest insects Britain

It looks like a mayfly or Damselfly.

I think we have a winner.