You Do Need power plug adapter while traveling from Namibia to Channel Islands (Guernsey & Jersey).

Here is Why You Need An Adapter

The outlet and plug type used in Channel Islands (Guernsey & Jersey) are C and G and in Namibia are D and M. This is how they look like.

Namibia

In Namibia, outlets of type D and M are in use.

Type D

Type M

Channel Islands (Guernsey & Jersey)

In Channel Islands (Guernsey & Jersey), outlets of type C and G are in use.

Type C

Type G