Hellooo and welcome to the forum 
First of all what clinic are you going to? I went to Holborn and was very pleased with their service and efficiency. My op was supposed to be yesterday but it got postponed because my throat swelled up like crazy..wahh
but they’ve managed to fit me in for 2 weeks time, so not long to wait 
Anyway, I don’t know much about uplifts, but I know a bit more about BA’s from what I’ve learnt on here and my own personal research….unders are when the implants are placed under the chest muscle and overs are placed over the chest muscle. Unders are seen to be the more ‘natural’ option (but overs can look natural too!) Unders can also be used for ladies with very little breast tissue to avoid them looking ‘stuck on’. Overs can create a ‘faker’ look if that’s what your after and the round part of the implant at the top can sometimes be visible…but it really depends on the person as to the outcome e.g your stats and how the implant sits on your chest wall. Unders tend to have a longer recovery time and are usually more painful as the surgeon will be making his incisions through the muscle as well as fat and skin.’Fluffing’ usually happens with unders, not overs as there is no muscle to compress with implants placed OVER the muscle. With overs, while you do have swelling, the skin tissue needs to stretch to accommodate the implants. Overs, will of course drop a little and the swelling will go down too but they don’t change as much.
I found this website very useful on the subject of size as it can be very confusing as some women, even with same size implants can look very different, so make sure to discuss what you want to achieve with your surgeon in as much detail as possible: http://www.poustiplasticsurgery.com/choosing-your-size.htm
Even though this site is American, they work with the same sizes/measurements as surgeons do here, so not to worry.
Hope this has been helpful and feel free to add me
xxx