WMG / Ultima VII

Britannia is a planet in the Wing Commander universe

The Kilrathi ship landed on Britannia because it looked like any other inhabited planet in space, it is only culturally in the past while the Terran-Kilrathi war continues out in space, just as described in Ultima Underworld II where the Kilrathi are known as the Trilkai. Otherwise, the ship comes from another reality or time period from a temporal portal or anomaly.