This Sale Agreed nonsense is just that, nonsense. Did I post that before?
The property is either "for sale" by the current owner or "sold" to the new owner, there is no interim status that has any legal meaning. The Under Offer, Sale Agreed, and Sold signs are marketing gimmicks invented by estate agents to show the world how outstanding they are at their jobs.
OP at no stage in the process do you have a legal obligation to have a sign outside the property saying anything - it's just an ad!