lapply(1:10000, function(B){
  set.seed(B)
  x <- runif(15, min = 0, max = 1)
  a <- ks.test(x, 'punif')
  b <- ks.test(x, 'punif', min = min(x), max = max(x))
  c(true =   a$p.value, est=   b$p.value)
}) -> boot

library(ggplot2)
dfs <- do.call(rbind, boot) %>% as.data.frame
dd <- melt(dfs)

ggplot() + geom_density(aes(x = value, colour = variable, group = variable), dd)