You mean in terms of showing income that you can support yourself? Alternatively it looks as though you can register as self employed without needing to show proof of income from the impression I get here:
Link:
Registering your residence abroad - Your Europe
You would need something to live of in the mean time between now & when you retire (savings, etc) or stay back in the UK for a while so long as its not for a period longer than 6 months as that will affect residency as I understand it. My thought is that you could register self employed there and of course you would not immediately have any income from that for them to expect of you. Whether you actually did any work self employed or not doesn't seem to be an issue. That is what I plan to do initially any way unless I have misread the situation?
That way you could get the yellow slip now so long as you gain a residency there which you can do within the 3 months should you wish or at least that is my reading of the situation.