Building a workspace

[Stones]

Attached is a plan of our single upstairs room which we use as a study office.  I currently use an old office corner desk but want to construct something more permanent so my desk sits under the window which affords a good clear view out (might not get any work done, but hey).

 

Mezzanine.pdf

 

The room is 3.5 metres long, and I want to return around each corner by about 1.6 metres to create a shallow U shape.

 

Ideally we would like to have an oak finish, and the obvious choice would be oak kitchen worktop.  The problem is the cost of doing so due to the additional haulage costs I have to pay.  An alternative is to build the actual structure with plywood, then top with either MDF veneered in oak, or with engineered oak flooring.  The biggest issue that I can foresee is that of deflection - I am aware that single spans of 18mm ply or MDF would be limited, although a sandwich of the two glued together would presumably offer more scope for unsupported lengths.  Likewise I know oak worktop would need some support, albeit at 40mm thick, less than the ply or MDF used on their own.  What is the maximum length of span before some form of support is required and how best to provide support without having support legs at the front?