You can easily flex the head 0.010" when you tighten the head bolts.The amount of torque on the bolt to cause that would be minimal. You could probably bend it 1/2 that in your hands with finger pressure. The head will then sit flat against the gasket and seal OK.I read the Britbike forum and didn't really see any cons on skimming the head, just the alternate to bend it back to normal.
.005 seems like such a small amount of warpage. Wouldn't the head gasket take that discrepancy out?
I've seen heads with 0.010" bow that still sealed OK.
Don't let them mill the head. Get that in writing, that they will replace it if they mill it. Measure the overall thickness, so you and they have a record of it. Have them put a tag on it that says "NOT TO BE MILLED OR RE-SURFACED". They will probably still mill it, as others have found out the hard way.