Just a caveat: what alcohol does to your blood sugar has a lot to do with what you're drinking and how much of it you drink. Alcohol itself does lower BGs through its effect on liver function, but some wines and beers have a high enough sugar content that it counteracts this effect and gives an overall rise - and of course ciders and a lot of mixed drinks can be quite sugary.
However, champagne is quite low in carbs as drinks go, and 30-50g of dessert is not a whole lot either (I think you meant that the dessert itself weighed 30-50g, not that it contained 30-50g of carbs, right?). It's likely that the dessert is the culprit in the slightly higher fasting the next day. But I think something to be aware of is that even this higher fasting, although shocking for you, is in the goal range (that is, the ideal range) of fasting BGs for a diagnosed diabetic, which is 70-130. Now, if you were seeing a consistent upward trend in your fasting BGs such that you're approaching the upper end of that goal range, that might be more indicative that you needed a change in treatment plan. Instead, what I'm seeing is that you generally meet ideal targets for fasting, post-prandial, and A1c, and you had a bit of a blip one day after eating some chocolate the night before (a blip that wasn't even technically out of goal range - which doesn't make it any less shocking for you, compared to what you normally see, but it's not anything to panic over just yet!)
I think it is possible that people become more carb sensitive after low-carbing for a while. Your body has "switched over" to not having so many carbs to process, and when you "switch back" temporarily it can be caught off guard and need some time to ramp up that processing ability again. However, again, what you're seeing is a reading that is still within ideal targets (if higher than normal for you), so I'm not sure that we can really consider this as a "high reading" that indicates you're more carb sensitive. Bear in mind that BG can fluctuate quite significantly from moment to moment, so you may have seen a somewhat different number just minutes (even seconds) before or after you got that 122. Meters are also not infallible and can have a margin of error of up to 15%. So it's important not to put too much stock in any one single number result, but to look at trends and patterns over time.
Another thing to bear in mind is that BG can be affected by loads of other things than just what you eat, including but not limited to: the weather, stress (mental/emotional and physical), illness or oncoming illness, injury and healing, exercise (can make BGs go either up or down, it depends on the circumstances), overall activity levels, medication absorption, and so on. So seeing a higher-than-usual number on your meter one day may have been a fluke of the test itself, may have been down to what you ate the night before, or may have been down to any number of factors that you may not even have been aware of at the time.
So, as regards the Newcastle diet, looking purely at your results, I think (though I am not a doctor or a dietitian) what you're doing now is working and I don't really see that you have any desperate need for a change in diet or treatment plan at this point, unless you feel that what you are doing now to achieve these results isn't a sustainable way of living for you in the long term? If the issue is that you struggle with wanting to indulge, especially when you go out (something I think we all face!) a *more* restrictive diet may backfire and increase your desire to go "off piste" rather than curb it. On the other hand, you know yourself best and if you function well within a more rigid structure then it could be that the Newcastle diet might provide you with that structure.
I'm not sure you fit the profile as you aren't recently diagnosed and you don't need to lose 10-15 kg (in my non-medical opinion), so you may not get the expected results. But in the end, no one can really say whether it might benefit you or what might happen if you tried it; the only way to know that is to actually try it, but ideally you should consult your doctor or a dietitian first and get some more official guidance on how to go about it, or whether there might be an approach to diet that would be more suited to your needs - something that gives you routine and is easy to stick to. If you do want to go for the Newcastle protocol specifically, I understand that it's quite strict, so still probably best not to go it alone without any of your HCPs knowing, in case there are any adverse effects you weren't expecting.