Getting Good Cleavage Without "Uni-boob" Look

The implants whether placed under the muscle or under the breast tissue only, sit on the chest wall. If your chest wall slopes laterally (prominent breast bone), no matter where you place the implants they wiil tend to slide laterally. If your breast bone is depressed, your breasts can end up closer than you want them. If your chest wall is straight, than it is a matter of pocket dissection and implant base width. I like to leave a 2cm undissected area over the breast bone to avoid a "uniboob". Your niiples seem high due to overdissection of the inferior poles of the breasts or stretching of the tissues over time. This is also known as buttoming out. This might be correctable with a larger implant, but I would close off the inferior pocket overdissection to "bring" your nipples into a better position.

The base width of your breast will determine the diameter of the breast implant. You are correct that you are at risk for creating a "uniboob", (also called symmastia), if the pockets extend too medially over the sternum in an effort to fit a larger implant. Your breasts may naturally be spaced apart, making it difficult to create cleavage.
Regarding the position of your nipples, the implants may be too low on your chest wall. This can be revised.
Good luck!

If you are UK based, I will assume that you had silicone gel implants, as gels are widely used abroad. Here in the US we are starting to overcome the negative propaganda of the Moratorium, in which silicone gel implants were restricted. Given your original breast size "A" cup and transformation to a "D" cup, I think that your surgeon probably released the inframammary fold (the lower area where your breast initially lay against the rib cage), in order to make an adequate space for the implant. Occasionally, this maneuver can cause the distance between the nipple and the new fold to lengthen and make the nipples appear too high. In my experience an "A" cup usually corresponds to a nipple-fold distance of up to 5cm; "B" cup up to 7cm; "C" cup 8-10cm; "D">10cm.
In proposing that you upsize to an implant which is an additional 1.5cm in base diameter, your surgeon is probably attempting to use the width of the implant to fill in the gap between your breasts. I think that this range 385-420cc would be reasonable, as long as your implants are situated in the dual plane (i.e. subpectoral). The risk of "uniboob" or symmastia increases as surgeons attempt to satisfy the patient's request for greater cleavage and over dissect the inner lower quadrant. An ideal distance between the breast mounds should be on the order of 2.5-3cm. Repair of symmastia is technically difficult but possible. On the other hand, if your implants are on top of the chest wall muscles, your risk of symmastia is higher with placement of wider implants. Consider your options carefully. Good luck.