The center text reads "There is no deity except Allah, He is the one and only, He has no partner." The text around the circumference reads "Muhammed the Massenger of Allah, sent him with guidance and true religion to make it victorius over all (other) religions." Obverse: The center text reads "Muhammad is the Messenger of Allah with the name of the King Offa Rex written upside down within that text." The text around the circumference reads "In the name of Allah. This (coin) was minted in 157 A.H." (157 A.H. corresponds to 773-774 C.E.).
The coin shown in the image was minted by the Anglo-Saxon King Offa Rex (757-796 C.E.). It was discovered in 1841 C.E., and is displayed in the British Museum. This coin is an imitation of Muslim dinar in circulation during the eighth century.
Before England was united, the Anglo-Saxon King Offa Rex ruled the area called Mercia (Middle England). Offa made important pacts with Charlemagne and Pope Adrian the First. Offa’s daughters married the rulers of Wessex and Saxon which facilitated strong relations between these areas.
Al-Kalby is of the opinion that King Offa became a Muslim. He suggests that Offa may have gone to Spain to study the religion or the culture of Muslims and may have become impressed by the Muslim civilization and its grandeur (Quotations on Moorish Civilization). As a result, he may have accepted Islam by his own choice. King Offa did not conclude any peace treaty or a trade treaty with either Islamic Spain (Andalusia) or with Abbasid Caliph in Baghdad.
About twenty years before King Offa established his kingdom, Gothic Princess Sara married 'Isa ibn Muzahim, one of the Muslims at caliph Hisham's court in Damascus and returned to Seville with her husband. Sara was the granddaughter of Witiza, penultimate king of the Visigoths. The achievements made during the reign of Sultan Abdur-Rahman, the 'Falcon of Andalus' (755-788) are well documented and this period marks the beginning of Moorish civilization. It is about the same period when King Offa ruled Mercia. Martin Hume in 'Spainish People' writes that Sultan Abdur-Rahman was one of the Heaven-sent rulers of men, his domain was the best organized in Europe, and his capital the most splendid in the world.
The Muslim creed inscribed on this coin minted by King Offa raises a great difficulty for those people who try to explain away these coins using a variety of arguments while pretending that Muslims did not exist in Spain, let alone rule it with grandeur better than any in Europe. [A Chronology of Muslims in Andalusia].
The source: http://www.cyberistan.org/islamic/offa.html