So I bought a polaroid in a charity shop for £5 (I know absolutely nothing about cameras, and I mean NOTHING) but it doesn't have the plastic bit inside the main round bit- I think that's a lens? How drastically will this affect photos before I start buying film? Will the photos just come out white or unfocused?
From the pictures and your description it seems that the lens (the spherical part in the middle) is missing.
Taking photos without the lens will not give you any sharp pictures, all you will see is some very blurry colours.
As Polaroid film is rather expensive I would not waste it to this partial camera, but would look for a fully functioning one.