Would a French victory at Trafalgar by itself make an invasion of Britain possible?
Short answerNo. France would also have to defeat or neutralize the Channel Fleet and secure the waters needed for an invasion.
NotesA French victory at Trafalgar would still leave the Channel Fleet to overcome. The force engaged at Trafalgar was only part of the Royal Navy. Changing that battle's outcome therefore does not establish control of the Channel for an invasion.