I'm afraid this will be difficult to answer, because one person's "folk" may be another person's "pop" already (it comes down to the old unanswerable question "what is folk?")

To say that singer/songwriters are pop singers is a bit sweeping. I guess you could say that Ewan MacColl was a "singer/songwriter" because he wrote and sang his own songs, but few (or nobody) would call him a pop singer.

But I guess you are thinking more of the guitar-playing singer-songwriter, that we sometimes see in folk clubs? I admit some of these I wouldn't say are folk singers because of the material they write and sing. I think the material they sing will define whether you think of them as a folk singer or not - but I'm not sure where you draw the line.

Then again, some 'respected' folk singers occasionally write/sing songs that taken in isolation would probably not be considered 'folk' songs. Again, I don't know where you would draw the line.

So, "What subject matter makes a song folk and what makes it pop?" is a question I'd like to get an answer to.

As for Slade, yes, I could imagine in 100 years that could be called a folk song, if you can imagine people singing it at a session round about Christmas time.