Thanks for advice. My mmol/L after 16 hr fast was 7.6 and 2hrs after lunch it was 4.8
If that's as high as you go then the prediabetes assessment seems right. They are still (just) within normal levels. The after lunch figure is excellent. The fasting one is consistent with something called the dawn phenomenon. Basically, your liver pumps out sugar first thing in the morning so you can get active (go and kill an elk, or whatever, in prehistoric times) and because your ability to handle carbs is compromised your blood sugar goes up in response. For many of the T2s on here the fasting blood sugar is the last one to go down when they treat T2 with diet.
I'm not the right person to ask about gaining weight while on a low carb diet, but there are thin T2s on here who will hopefully post soon. The good news is that you've got a meter so you can test before and after a meal to see what your body can cope with. As you're only prediabetic there's a good chance that a relatively minor reduction in carbs will fix the diabetes issue, but if you cut down on carbs you'll need to increase the fats/proteins. Bacon and egg for breakfast?
Good luck.
ps Edited to add. And it doesn't necessarily have to be T2, as you could just be producing a little less insulin because your pancreas doesn't work quite as well as it used to. But even in that case, reducing carbs should help, as then you'll need less insulin. But undesired weight loss is a concern for anyone, and shouldn't be ignored. How about snacking on a boiled egg, some cheese, or some tuna between meals? (There's plenty of other low carb snacks.)