This website is American but it has some useful information on size and what you should discuss with your surgeon; it explains how even women with similar stats and the same size implants can look completely different: http://www.poustiplasticsurgery.com/choosing-your-size.htm
Good luck! xx
PS as for D and DD I don’t think there’s much difference but I would expect that DD’s are fuller than D’s…I have a friend who is a natural D and she can fit into both D and DD cup bras, it really depends on who the bra is made by and the individual etc. Feel free to add me xx