ive just read back through the the thread and your talking about routing out the underside of the top to accept a washing machine
all tops have to be sealed all round as the chipbard will absorb water and blow the laminate off
the underside of your top has a paper stuck to it. this has two uses , one is a waterproof barrier to protect the chipboard core and the other is a counter laminate to prevent the top bowing, if you remove the paper the top will curl
routing twenty mm from the underside will leave you 8mm-18mm of top, this is insufficiant thicness for a kitchen worktop
we have in the past made removable sections over the fridge to allow the client to retrofit a larder fridge, these removable sections were screwed down to keep them stable, I dont see how you can fix down this section of top once you have installed the machine or how you will install the machine with the top screwed down
is it not possible to gain the room you need by removing the top of the machine as already suggested, the top is removable for this very reason