I doubt very much that Offa was a convert to Islam (and the promoters of Islam Awareness Week don't say that he was). Only one known gold coin of Offa's carries the declaration of faith, and it is not on his more numerous silver coinage. A public conversion would surely have left some stronger trace in the historical record. One can, of course, argue (as this article does), that later Christians have deliberately expunged all reference to Offa's Islam, but I don't buy that sort of conspiracy theory. I think it is far more likely that the coins were done in imitation of Muslim coinage for circulation in the Muslim world, created by coin makers who didn't actually know what the inscription meant.