I am no expert on these but I can tell you straight away that it is an English Groat or Halfgroat (Diameter would help clarify which).
King is Edward - I suspect Edward III, ("Edward" on the obverse) but more expertise to be sure. Minted in London - "Civitas London" on the reverse.
Value will depend on an accurate ID of the precise variety. It is fairly well worn and has been over-scrubbed in the recent past.