James,
The pistons are just plain WRONG mate, really..!

a, they shouldn't foul the head for ANY reason, variations in headgasket thickness etc shouldn't be that critical. The block hasn't been machined short, as it was OK before with the standard pistons.

b, the compression ratio isn't even worth discussing, it WILL be 10:1 as it stands, same as standard, since the pistons come to the top of the bore..

There's 2 possible explanations in my view here:
Either they machined the wrong pistons, so haven't ended up with the dimensions they were expecting, or they actually haven't been machined at all.

As a secondary issue I would also expect someone with the knowledge/expertise that Burtons should have, to machine pistons for lower compression with a dish/bowl rather than flat - it really isn't good practice to have flat pistons with a large flat combustion space between the piston and head at TDC.. this also adds to me thinking that they haven't actually been machined..