The Newcastle diet affects people in different ways, but is mainly a weight loss diet. It is only 800 calories a day. Some people have lost a lot of weight. Some people have found it puts diabetes in remission. Some people find it doesn't. Whatever the effect, once the 8 weeks are over it is necessary to maintain a diabetes friendly diet or run the risk of weight gain and higher BS levels.
Perhaps 5 months on LCHF isn't long enough for you. It takes some people longer than others. I have been on LCHF for over 3 years and have no weight to lose but only recently can I introduce a new carb with little effect on BS (and in smaller portions than "normal".) My current experiment is with baked beans with a fry up.