The folk here
http://www.safetysignsupplies.co.uk/default.as px?d=34 sell the signs you're looking for and if you click on the one you want, a price list of the various options comes up and a cheap one is an adhesive sign.
Another option is to cut and paste the image in question and blow it up to colour print it on A3 size for example and laminate it. You can practise inserting text until it matches the size of the blown up font and then as they say, Robert is yir father's brother!
I'll have a look in the meantime to see if there's a Road Closed sign in the Acts relating to road signs.