You Do Need power plug adapter while traveling from Great Britain to Vatican City.

Here is Why You Need An Adapter

The outlet and plug type used in Vatican City are C, F and L 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

Vatican City

In Vatican City, outlets of type C, F and L are in use.

Type C

Type F

Type L