You Do Need power plug adapter while traveling from Guam to United Kingdom (UK).

Here is Why You Need An Adapter

The outlet and plug type used in United Kingdom (UK) is G and in Guam are A and B. This is how they look like.

In Guam, outlets of type A and B are in use.

Type A

Type B

In United Kingdom (UK), outlets of type G are in use.

Type G

I am from

going to