The cavity is an air gap, originally put there to prevent damp on the outer wall migrating to the inside. Air does act as an insulator but foam is much better as it prevents air moving in the cavity - it is trapped in air bubbles.
Cavity installers on retrofit installations drill 20mm holes in a pattern typically one metre apart, then pump foam in, using the next hole as one means of judging if enough is inserted.
It could be a small void, or it could be the whole job was faked. If you can't see evidence of foam at the top of the cavity by the eaves, and there is no cavity closer, the job was faked.
To be sure, try drilling near one of these holes you found. No foam equals faked job.
You have no chance of redress really, unless from the guy who did the EPC for the previous owner before sale. What does the EPC say about wall insulation?