Because saltwater is going to escape the artificial lakes even if they have impermeable beds, and we'll, there's a reason “salt the earth” is something that historically people have done, or at least talked about doing, to mortal enemies’ lands and not their own.
Have you seen how small the UK is vs. our population density? I highly doubt you could find a site big enough and at the correct elevation to make a noticeable difference.
We're the size of Michigan with 67 million people.
UK is an island, and you have highlands. Why not make some salt water lakes there?