Encyclopedia Britannica hints -  (Britain ) -  p. 168.
In  Offa's  time a new  gold coin ,  the mancus resembling in standard the Roman Solidus ( about 70 grains) was introduced from Moslems,   the oldest  extant specimen bears a faithfully copied Arabic inscription  (= was written in Arabic)!
While in his book "Secrets behind their  Islam", 
 Kamel AbdSamad states directly  that King Offa was a British Moslem King and had his Islamic British golden  coin  with his name and Islamic faithful inscriptions on it.