If you have lost 5 stone or more, and kept it off, in some cases you can have a tummy tuck on the NHS.
Skinny with saggy skin = look good in clothes, look bad naked.
Fat with no saggy skin = look bad in clothes, look bad naked (not in all cases I know but I assume there's a reason you are losing weight that is aesthetic as well as for health).
You will fare better in the looks/aesthetics department when you've lost the weight.
If it makes you feel better, a saggy tummy wont really look saggy when you are lying down. You can still look good in lingerie with a saggy tummy.
Hope some of that makes you feel better.