In my opinion you are perfectly entitled to do so.
What you are not allowed to do is post an "abusive or insulting" sign - that would be an offence under the Public Order Act 1986.
In 2012 a pensioner in Lincolnshire caused a stir when he posted a sign in his window saying "Religions are fairy stories". This was reported in the press, see for example:
http://www.bostonstandard.co.uk/news/local/update-police-now-issue-statement-on-boston-pensioner-s-religions-are-fairy-stories-poster-1-3962839