Leave the pistons alone, they will look that way 2 months from now.
As for the block, a little carb spray and don't use anything abrassive to clean/scratch them as for the heads, have them hot tanked by the machine shop if you want them really clean.

 

Should I just scrape the deck with a razor blade? Heads are being done at the shop now

 

I use a new gasket scraper to lightly remove the heavy stuff being careful not to gouge the surface. Then I hit it with a 3M roloc surface conditioning disc on an air angle-grinder and give it a light pass. Some people don't like the idea but I've done many, many head-gaskets on cars and class 8 trucks with no issues ever. The reconditioning discs don't remove any metal so they're safe. I think people confuse them with other 3M roloc discs like the bristle discs (light sanding) and the coarse scotch-brite discs.

I agree though. Leave the pistons alone.

 

For cleaning the deck - I just used carb cleaner from NAPA.
Get rubber gloves for your hands.

This will get most of the carbon off of the deck.
I used the green pot scrubbers used in the kitchen with the carb cleaner.

Never ever use a metal object.

I did not have my heads done or inspected. I just flipped the heads back on.
But then, I have only driven 3,000 miles since March 2012.

I may regret for not getting the heads done.

But, I don't really trust shops that I don't know.

But, my feelings now about head gaskets on Discovery is that these are like shoes. They wear out no matter what.

Here are some stories about HG replacement on the Discovery I

1997 Land Rover replace catalytic converter and head gaskets

Here are some pictures for the Discovery II

Flickr: Landroverdude2's Photostream