If you don't plan on going back to the UK ever again, you could ignore it. But being unpaid it will compound, and may cause you problems at the border if you do ever go back. So just pay it, regard it as a relatively cheap lesson learned, and move on to more important things in life.

FWIW, I got hit with a fine from Florence, Italy, a while back for driving in a part of the city that you are not allowed to enter unless you are a resident. Saw no signs at all. So it goes.