Well, the belgian province is optional, but I think this area should be distinguished from the rest if only by making it part of Belgiae province using the same color, the way I described above. The more research I'm making the more I see how people living in this area considered themselves 'different' from the rest of the Britons...
With separate Belgians it's 4 new provinces, without - 3 new provinces. So it's not that much, I think we'd manage below the limit.