Choosing Artisan Naturals Kingsize mattress for different weights

Hi, I have been looking at your website for some time. I think I am ready to order an Artisan Naturals Kingsize mattress, but I know that previously I have read you saying to get in touch first. We have a slatted bed. I weigh 13.5 stone and my wife weighs 9 stone. We both like a medium feel, but I guess that means tuning the mattress for our weights? How do I specify this when ordering?