Slimming World is a very healthy, balanced diet, but also low-fat so you may need to be careful. I personally find that now if I eat a high-fat meal (even if it's just lots of healthy olive oil), I do get stomach pains and feel sick, and I've heard that this may be because the gallbladder is not used to oils any more and works more slowly. So when I get to my target, I am going to gradually reintroduce healthy oils like olive oil to avoid future problems.
Maybe talk to your doctor about ways you can adapt your diet to prevent gallbladder problems in the future - whether it would be best for you to avoid fats, or to include them (eg olive oil as a healthy extra B). I think most doctors would agree that unless you need to lose a lot of weight quickly to save your life or something, a VLCD is not the way to go and something like SW is more likely to be recommended.