The original London planetarium has been relegated to showing Marvel superheroes (the poms are losing their sense of what Really Matters IMHO) however there is another planetarium at Greenwich
http://en.m.wikipedia.org/wiki/Peter...on_Planetarium
If your interests extend to Victorian engineering you can't miss the science museum and the Brunel museum. I'd suggest read a biography of Brunel first in case you don't know - and consider travelling to see some of the landmarks he created, many are still standing in daily use.