When Britain was thus dismembered from the empire its importance was sensibly felt and its loss sincerely lamented. The Romans celebrated, and perhaps magnified, the extent of that noble island, provided on every side with convenient harbours; the temperature of the climate, and the fertility of the soil, alike adapted for the production of corn or of vines; the valuable minerals with which it abounded; its rich pastures covered with innumerable flocks, and its woods free with wild beasts or venomous serpents. Above all, they regretted the large amount of the revenue of Britain, whilst they confessed that such a province well deserved to become the seat of an independent monarchy.
In this passage, Gibbon seems to get a bit carried away with some of the panegyrics written after the recovery of Britain from the rebellion of Carausius. He occasionally has a tendency to get a little bit patriotic and play up his home in a way that jars with the rest of the narrative.
It’s a good thing everyone has stopped over-estimating the importance of Britain!