The main drive for the Islamic expansion out of Arabia was primarily demographic. The available land was not enough for an expanding population, along comes a uniting religion and off you go and knock over the war exhausted local empires! In this timeline the eastern empire is stirring the pot in the successor states of this western Roman Empire prior to an invasion to retake them for the empire!!!! Ala OTL really but including Britannia this time! Even if islam is butterflied away I think that the Arab invasion will still happen. that the stirrup will have made its way to both the arabs and the Persians by now is I think a given. This would have massive effects on these areas due to the reliance on horses for the basis of military might (in part due to the lack of rivers and the distances involved if nothing else) so it's roll your dice time!