Britain became a Roman colony in AD 43 under Emperor Claudius. However, the Celts had close contacts with the Romans long before the invasion. About a hundred years earlier Julius Caesar organized two expeditions two Britain (one of which was in 55 B.C and the second one in 54 B.C). The Celts also fought in Gallic wars and imported wine and other luxuries from Rome. この表現は自然ですか？

Britain became a Roman colony in 43 AD under Emperor Claudius. However, the Celts had close contact with the Romans long before the invasion. About one hundred years earlier, Julius Caesar organized two expeditions to Britain (the first of which was in 55 BC, and the second in 54 BC). The Celts also fought in Gallic wars and imported wine and other luxuries from Rome.

So what you already have is fantastic, there's just a couple of little tweaks to make it completely natural-sounding. The first is that we place the time marker (AD, BC) after the number, instead of before. I think you know this as you had them in the right order in the 3rd sentence. "Close contacts" in the second sentence is slightly incorrect, the correct version I provided above. As I'm assuming this is a formal piece of writing, the term "a hundred" isn't quite correct for the tone of the piece, which is why I corrected it to "one hundred" (although both are technically grammatically correct). In this sentence, I also changed the way you phrased the parts in the parentheses, as the version I provided sounds more natural.

I hope I was of help!