You Do Need power plug adapter while traveling from Guinea-Bissau 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 Guinea-Bissau is C. This is how they look like.

Guinea-Bissau

In Guinea-Bissau, outlets of type C are in use.

Type C

Channel Islands (Guernsey & Jersey)

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

Type C

Type G