Stepping back a bit, I was thinking that the bunch on the Titanic were from a similar outfit as the tourists from "Delta and the Bannermen". In that serial, the Seventh Doctor and Mel hitch a ride with a group of aliens going on a tour to Disneyland in 1959. The aliens are actually purple tentacle monsters (ish) that we actually see, but transform themselves to look human, and likewise time-travel to Earth in a space cruiser designed to look like a bus from the 1950s.
While Doctor Who is replete with human-looking aliens, I don't see why some of them don't just change themselves accordingly when they drop by. Some like Banakafalatta would avoid the process (lest he be revealed as a cyborg) but everyone else would make a temporary switch so those puny Earthlings don't think they're being invaded AGAIN or something.