I can't think of a physiological reason why you couldn't get all of your 5 a day from smoothies, as long as you drank the smoothies over the course of the day to prevent blood sugar spikes. I think the NHS tells you that so that you don't drink too much at once & get sick or so that you don't have blood sugar spikes. However, what they say about sugars on teeth seems valid, so it'd damage your teeth more. Course, if you brushed your teeth afterwards you'd be okay, but so much effort to liquefy foods! I add kale or spinach (baby spinach is even better, you can add more without it tasting different) to my smoothies sometimes to boost it's nutritional value; it's taste is covered up by the fruit.