score:2
From TIMATIC, the database used by airlines:
Nationals of Colombia are allowed to enter with an expired passport.
and
Passport Exemptions: Nationals of Colombia with a national ID card
So yes, an expired Colombian passport (or, alternatively, a valid cΓ©dula) is sufficient.
To exit Colombia, I was told by Colombian immigration on the phone a while ago that a valid cΓ©dula is sufficient, but not an expired passport.
Basically, bring your UK passport and Colombian cΓ©dula on this trip