Roger Paul had already identified this coin as hammered silver penny of Edward III. However, I was asked for my opinion as to what it could be worth.
There is a quatrefoil in the centre of the reverse, which indicates that it was struck for the profit of the Archbishop of York. The bust on the obverse isn’t altogether clear but it appears to have a fleur de lis on the breast. Therefore, this silver penny will have been struck during the post treaty period of Edward III for Archbishop Thoresby or Neville. In the Standard Catalogue the type is listed as number 1649.
The obverse would grade about Fine, the reverse only Fair. It’s a very scarce coin but in its present condition I’d price it no higher than £15 – £18.