This coin has been identified at the British Museum.

It Appears to be an Iron Age Bronze coin from North West France.

The type appears to have part of a head on the front (although it is difficult to see as it is struck so off-centre).  On the back there is a serpent-like creature above the horse (facing right) and a little boar below the horse.  It is difficult to be 100% due to the problems with the centring of the obverse, but it looks like a type proper to the Somme/Oise area usually attributed to the Ambiani tribe.  The type is published in Scheers 1977 volume Traite de Numismatique Celtic. II. La Gaule Belgique, Series 63 or Delestree & Tache 2002 volume Noevel Atlas des Monnales Gauloise.  I. de la Seine au Rhin, Series 46, No. 362.

Found in Oxfordshire      Weight: 2.28gms       Size: 16.05mm

Fine