Seriously - DON'T BOTHER M8
It is a cheapy plastic CM513 about £70
You would end up buying a keymod which might fit ok but cost say £40 or more
It might not fit on the thread - think Mr B or Monty had a bitch of a time with an old Cyma (metal) receiver
End of the day use the gun for what it is a cheap but good value starter gun
Save your money and put it towards a better gun down the road imho....
OP - remain within the law but if you acquire a defense then just "customise" it a little bit is what I'd do
You could add a bit of camouflage tape as a temporary measure at the site
(again you need to stay within the law but I'm not getting too deep into this subject)
It isn't worth spending much money on it - heck you could buy a new CM515 keymod M4 for about £67
(once you get UKARA etc....)
Play safe, stay legal & save your cash - hope some of this helps