I strongly suggest that you read the Act I referenced in an earlier post (sections 90-odd through 100-odd), which clearly states that a parking contract will be formed by the presence of signage.
Even if you choose not to believe that to be the case now (and I have provided a link to a commentary on a Scottish case, written by a Scottish lawyer that says it is the case today), it absolutely will be once the relevant regulation is published by the Scottish legislature.