First of all, Wallace never had a romance with Isabel from France (portrayed by Sophie Marceau), since she only arrived in England three years after Wallace's death. And Longshanks - that was Edward the first's real nickname, along with Hammer of the Scots - wasn't dying when William was executed. He actually died in the battle of Burgh-a-Sands (Cumberland) in 1307, near the Scottish frontier, on his way to another campaign against Wallace's friend Robert Bruce.
Finally, it is most unlikely that Wallace was just a commoner. It is said that he spoke several languages and was a cultured man - as it was shown in the movie -, but that makes historians believe that he and his wife Murron (who, by the way, was no secret bride) were part of Ellerslie's aristocracy.