The exact wording of the restriction is:
<font face="Verdana, Arial, Helvetica, sans-serif" size="2">You are not eligible to donate if, since 1980, you spent a total time of 3 months (UK - 6 months rest of western Europe) or more in any of these countries</font>
I'm not too sure if this is cumulative or not. However, in Canada the rules are more restrictive - not only is it a cumulative total, but it's 3 months for both UK and France, and 5 months for other western Europe countries.