This coin came in from a detectorist who signed himself as Wayne. It looks to be massive but is actually only about 12mm in diameter, so it is a halfpenny dating from the later medieval period.
Both sides have been struck off centre and this can make things difficult for ID purposes. Additionally, there are no extra marks (saltires, pellets, annulets, etc.), which are often helpful.
On the obverse the legend ends with EX and starts with EDW and it is in the style of Edward IV. The mint mark is larger than usual and is a cinquefoil with an open centre. On the reverse the legend isn’t altogether clear but if the whole of it showed up then it would read CIVI TAS LOn DOn. Therefore, this halfpenny is an example of type XXI, which was struck during the second reign of Edward IV. In the Standard Catalogue it is listed as number 2137.
As already mentioned, both sides are struck off centre. It is also weak in places and the edge has been bent up in one quarter. On the plus side enough detail shows up for a full ID. In terms of value, in its present condition my price range would be £15 to £20.