Getting into ketosis is the difficult bit, so I never have a day off, as I don't want to have to climb that particular mountain again! You could do SS+ and you can eat protein, which you could be a bit more flexible with and have some protein based meal if you went out. Also you could have coffee/tea with milk too.
I have chicken every day as my meal, and I put salt and a bit of mayo on it! Not that that's in the plan or anything, but it's fine. I also don't weigh my chicken and eat according to how I feel that day. Some days I just have a small plate, others I'll have two huge plates full! Anyway, it's not affected my weight loss. Also, I will have two cups of tea of an evening with skimmed milk, but the rest of the day I drink black coffee, which I love now. I always thought that there was no way I could drink black coffee, but honestly, I can't bear the thought of drinking coffee with milk in it any more!
I like Cambridge as it's a bit more flexible than LighterLife (and much cheaper) but it also has it's specific rules. I know that I bend them ever so slightly, but my weight loss is great and those little things I tweak help me stay on the diet and actually enjoy it. With LL I didn't eat a single thing for 20 weeks, so I know I have the willpower. Although when I did WW, I would have a 'weekend' off and then say I was getting back on the diet. It's so much easier to not have those treats on Cambridge, and the big losses also keep you motivated. That's not to say that people don't 'cheat' and eat carbs and stuff, as they do, but it's all down to personality I think. I'd give myself such a hard time if I 'broke' the diet, that I stick to it!
Definitely try it, you've nothing to lose. Oh, and someone commented about the cost - it's definitely not expensive to do as you won't be buying wine/takeaways/lunches in work, etc!