Ask the builder if you have chance - they all dab differently.

Many are happy for you to fix a 25mm box directly on the wall, where others want it sunk half way in.

As for the cable - there is nothing to stop you simply clipping it tight to the wall, but this is considered a little DIY-ish.  Capping is the next best thing, directly onto the wall.  A proper job would be oval tube sunk in slightly, but this has become pointless.........as you would never win a contract if you priced to chase every job a smidge.

Upto you - clip the cables tight, or cap them.

As for the box - ask the builder how far the finished wall front will be from the block.