Personally, I think it is possible to lose a stone or so in 6-8 weeks by following less restrictive diet plans. I do Slimming World and, while I haven't been particularly good lately, I did lose 8 lbs in the first 2 weeks, no change for 2 weeks (well, put a couple of lbs on and lost them the next week) and this week I reckon I have lost another 2 or 3 lbs. Not a bad result really for 5 weeks.
I couldn't do anything as extreme as Lighter Life or Cambridge, purely because I do like my food and I'm not sure a shake or soup 3 or 4 times a day for ever more would be something I could stick to.
However, from what I've read about these 'extreme' diets, you are carefully monitored - if you feel like you could stick to it then go for it (however, I think the very fact that you are talking about a 'day or night off' here and there suggests this may not be the right diet for you).