You Do Need power plug adapter while traveling from Great Britain to Åland Islands.

Here is Why You Need An Adapter

The outlet and plug type used in Åland Islands are C and F and in Great Britain is G. This is how they look like.

Great Britain

In Great Britain, outlets of type G are in use.

Type G

Åland Islands

In Åland Islands, outlets of type C and F are in use.

Type C

Type F