It is neither a policeman or fancy dress!!
It a British soldier, probably infantry of around the 1880. The helmet is the blue cloth "home service helmet"
There appears to be a spike on the top of the helmet.
I am undecided as to whether he is a bandsman or not....It may be a music case around his chest, and I am unsure without blowing it up (I'm at work and unable to do such fancy things!!) whether that is a bayonet or a bandsmans sword..

What would be handy as we cannot make out the helmet plate, is a close up of his belt buckle!!
