My parents are seperated and I never needed a letter from a solicitor.
In any case, as a solicitor has never been contacted by my pearents, it would mean that one or other of them would have to visit a solictor, who is probably a stranger to them, tell him they are seperated and then he writes a letter to tell the Student finance people that they are seperated, which is stupid.
Get your parent to write a letter saying that they are seperated but that they haven't begun divorce proceedings and don't have an official seperation agreement.