"The Colossus does not count"
The reason that the Colossus does not count as the world's oldest working computer
is simply that the machine on display at Bletchley Park is only about 10 years old.
There are other replicas and rebuilds of early machines around such as
the Manchester Baby, the Atanasoff machine and the Zeus Z3, the WITCH
however is original and is, AFAIK, the 3rd oldest surviving computer, the
oldest being Australia's CSIRAC and the 2nd oldest the Pilot ACE in the
London Science Museum.