Hi there. This isn't the DUK site, but in reality, how you choose to trim up (assuming that's why you would be joining Slimming World or Weight Watchers is entirely up to you.
Many on here have found great success by trimming back the carbohydrates in their diet, however, one thing I have to stress is if you intend to change your diet significantly, it is possible, even probable, that your insulin requirements will change.
If you are not comfortable adjusting your insulin to match what you eat, or if you are on a mixed insulin, I would strongly suggest you speak with your Doc or Nurse before making changes.
Hi and welcome to the forums. This is the DCUK forum, NOT the Diabetes UK forum, and I am not up to date with what Diabetes UK are currently advising.
Many people on this forum, myself included, have had great success with low carbohydrate diets (it's not low calorie). It's a very different approach and you might like to look in the "Success Stories" threads on this forum to see how people have benefited.