That trip to Japan was unusually long.

The Doctor being an existing well known actor is more forgivable to me, as that's the character who has to carry the show... so it is best to have someone who is known can act or attract ratings. I guess I just feel that the stories in Dr Who should be good enough to not need famous people in it, and if they have more than a quick cameo it is distracting (like the Christmas episode that had Kylie Minogue in a major role).

Billie Piper is a pop star? Must be just in the UK, as I've never heard of her from anything else.
I just had a look... she did quite well for what she released (even getting a few songs into the top ten in Australia), but then suddenly stopped in 2000 after just 3 years. I guess at the time of her role starting on Dr Who in 2005 she might have been best known for her singing, as her acting resume only really started in 2005.

As disappointed as I was in this episode, it was still better than the 2017 Power Rangers movie that was on last night as well. I'm glad I didn't pay money to watch that in a cinema....