Dieting app. The problem as you’ve described definitely exists. However, there aren’t perfect solutions. Recognising food (which is the biggest problem here) just by taking a picture is unfortunately impossible (there are thousands of ways to cook a chicken, and everyone’s way of preparing it is going to be different). So until our mobile phones come with a spectrometer (which is possible and would make this problem a lot more solvable), there are no readily available technical solutions.
That being said, and app that locates when you in a restaurant, and pulls restaurant menu from website associated with Google Maps record of the restaurant, and asks you what you ate/ or proposes you to view menu version with taste/ dietary comments from people who already at the place, could be extremely useful, and definitely feasible.
Subleasing is less of a problem in UK. There is spareroom.co.uk which is the most popular way to rent your room. There are options that distinguish short-let and long-let. However, there could be a website that better distinguishes a short-let from long-let, e.g. subletting often means that the subletter is taking upon himself a far greater legal risk & there is no framework for taking a deposition. A website that adds legal framework, insurance, and deposit would certainly be welcome.
Regarding the cafe, there is already https://ziferblat.co.uk/#/.