You Do Need power plug adapter while traveling from Montenegro 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 Montenegro are C and F. This is how they look like.

Montenegro

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

Type C

Type F

United Kingdom (UK)

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

Type G