The neighbour of our polling station has a wall that overlooks the forecourt of the polling station, and they've put a big poster up for one of the parties. So everyone going to vote will see it on their way in.
It certainly isn't allowed inside the polling station but outside on private property is different. If you were unhappy you could bring it to the attention of the Presiding Officer and let them decide whether anything needed to be done
I don't think they can do anything if it's on someone's property. When the independence referendum happened the house next door to my polling station had posters, billboards flags and all sorts in their garden.