It looks as if you have an Emperor.
There are three Emperors to look out for in the UK; one of them, the Emperor Dragonfly/Blue Emperor (Anax imperator), is common and widespread whilst the other two, the Lesser Emperor (Anax parthenope) and Vagrant Emperor (Anax ephippiger), are less common though do get reported every year.
Avoiding colour variations which can confuse, here are markings to help identify them.
Emperor Dragonfly/Blue Emperor (Anax imperator)
Lesser Emperor (Anax parthenope)
Vagrant Emperor (Anax ephippiger)
Given a “normal”, i.e. mature, specimen, switching to colour from monochrome should be safe and could be clearer:
Emperor Dragonfly/Blue Emperor (Anax imperator)
Lesser Emperor (Anax parthenope)
Vagrant Emperor (Anax ephippiger)