I have searched in Eden's memoirs covering the years 1951-1957 but I have found no mention of the plan. However he did make this speech on 11 January 1952 on Britain's relationship with the rest of Europe:

If you drive a nation to adopt procedures which run counter to its instincts, you weaken and may destroy the motive force of its action...You will realise that I am speaking of the frequent suggestion that the United Kingdom should join a federation on the continent of Europe. This is something which we know, in our bones, we cannot do...For Britain's story and her interests lie far beyond the continent of Europe. Our thoughts move across the seas to the many communities in which our people play their part, in every corner of the world. These are our family ties. That is our life: without it we should be no more than some millions of people living in an island off the coast of Europe, in which nobody wants to take any particular interest.

From this I would guess that Eden's opposition to Mollet's proposal for a Franco-British Union sprang from worries over the implications to Britain's world role if Britain were tied to a Continental power.