Julius Caesar is said to have written about trying a fermented apple drink produced in the south east of England when he tried to invade in 55 BC – facing off against the Celtic tribes that lived there at the time. Though the first invasion attempt was a failure, the discovery that the local tribes were fermenting apples was a discovery taken back to France by Caesar’s retreating troops.
This recorded information would suggest that, as Caesar had already conquered France, he hadn’t at that point discovered cider. If they were making it in France, it was not a common drink. The Romans had not at this time brought modern apples, the Malus pumila, to England. This occurred when the Romans finally successfully invaded almost 100 years later, from 44AD. However it was not until ~1080 apples more suitable for making cider were brought from Normandy to England
So, this would suggest the cider like beverage Caesar tasted on the shores of England was made from the local crab apples.