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

Niger

In Niger, outlets of type C, D, E and F are in use.

Type C

Type D

Type E

Type F

United Kingdom (UK)

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

Type G