Not only was sea level higher when the Romans were in Britain, they also grew massive grape vinyards due to the warmer temps, aka Medieval Warming Period.
The medieval warm period was 950-1250 AD. The Roman Warm Period was 250BC-400AD. Roughly the 1st Punic War almost to the Goths sacking Rome in 410 for the Romans.
The medieval warm period corresponds to Erik the Red in Greenland to just past the 4th Crusade.