The chance that you have CHF without any typical symptoms at your age is so remote it's almost impossible to calculate. Go see your doctor, tell him your concerns and mention the blood to be safe but don't worry about heart failure unless you are so weak you can't walk or sit up, can't breath without oxygen and are lightheaded and dizzy.
I'm sure you're fine but go see a doctor to get some peace of mind and help get your weight under control.
You are only 18 years old & you know that your not healthy. You can take control of your health right now. You can start by eating healthy & walking every day. You will start feeling better soon. Go see your doctor and get checked out & get advise on a diet plan & exercise. You can do it! Do it for you! :)
You have already admitted some of your faults, such as unhealthy diet and lack of exercise. Add these to your stress of worry, you have created a passport to take ownership of heart disease in the future. Your weight (20 stone) is (as you said) way too much and so you know the problems. The choice is yours and nobody can force you to make it, but if you don't want heart disease at the age of 40, you can do something about it now.You don't say, but I hope you don't smoke? Your body gives back how you treat it, so you need to be nice to it and respect it. Give it lots of fruit and veg and exercise it regularly. Just start with a daily walk of 30 minutes minimum. It would probably be a good idea to get a Doctor aware of your goals and they can keep an eye on things with regular blood tests.