I believe the DVLA came up with their requirements following consultation with the NCC (National Caravan Council), which is a body of caravan/motorhome manufacturers, retailers and representatives from the Caravan Club (now Caravan & Motorhome Cub) and the Camping and Caravan Club.
As for the length of a 6' bed and indeed the fitting of a fixed table, yes they are down in the requirements, however I have been led to believe that the DVLA are more interested in whether or not the conversion being submitted to them for reclassification, actually looks like a Motorhome/Campervan as Darren so rightly points out.
As for me, my 'Millie' doesn't have a fixed table in the living accommodation as their requirements don't finitely say it has to be fitted actually inside the living area, my table is actually fitted into a securely retained fixture inside the doors of the storage shed at the back of the van!
We rarely use the table as we prefer to eat off trays on our knees, but when we do use a table, it is normally laden!