Adding scale bar to ggplot map

There is a package called ggsn, which allows you to customize the scale bar and north arrow.

ggplot() +
  geom_path(aes(long, lat, group=group), data=worldUk, color="black", fill=NA) +
  coord_equal() +
  ggsn::scalebar(worldUk, dist = 100, st.size=3, height=0.01, dd2km = TRUE, model = 'WGS84')

Another option is annotation_scale from the ggspatial package. Some more hints on this blog post.

library(ggplot2)
library(maptools)
data(wrld_simpl)

world_sf <- sf::st_as_sf(wrld_simpl)
worldUk <- subset(world_sf, NAME == 'United Kingdom')

Plot the layer and add the scale bar.

library(ggspatial)

ggplot() +
  geom_sf(data = worldUk) +
  annotation_scale()