Encourage A4 to page break before a tune. This enourages page breaks to happen before tune start, and not between the tune image and any following wording. Also move the vfill to the tune start. That way, if the break does not happen, the vfill will, and the following tune will be spaced down to the bottom of the page if it's the only other tune on the page. Otherwise everything tends to clump at the top of the page.

X:1
T:Sloe, The
S:Nick Barber
M:4/4
Q:140
K:D
A2| "D" d3d d2 "A" e2 | "D" f2>e2 d2c2 | "G" B2A2 BcdB | "A" A2>G2 "D" F2A2 |
    "D" d3d d2 "A" e2 | "D" f2>e2 d2c2 | "G" B2A2 "A" B2c2 | "D" d6 :|
A2| "D" B2A2 F2>A2 | B2A2 F2>A2| "D" d2c2 "A" c2B2 | B2A2 "D" A4 |
"A" d2c2 c2e2 | "D" e2d2 d2f2 | "G" f2e2 "A" B2c2| "D" d4 d2 :|