For casual indian street food try Roti Chai which is just behind selfridges. The upstairs room is great and informal. Selfridges often have a rooftop cafe during the summer which is usually best at breakfast time (before the posers arrive). Both are very near Marble Arch.
Marylebone has great restaurants (much better than st Christopher’s place which is more touristy). Caffe caldesi, ivy cafe, trishna, providores, woodlands, il baretto are all lovely. I’m veggie, but have heard great reviews of entrecôte for steak frites.
If you plan to go for Lebanese on Edgware Road, my favourite is Al Arez and my Lebanese friends all approve. Forget the mains and fill up on the Mezze (exceptional baklava too). I’ve never spent more than £15 per head there.
For street food, I would advise soho and just wander down frith, Greek, dean and wardour Street - you will be spoiled for choice. Princi is a super option (the pizzas by the slice are divine and if you get a window seat it is nice for people watching)
Oh how I miss living in central London 
- so many options for gluttony!
If you are planning a fancy meal, the reviews on hot dinners are pretty reliable and I like the cheese and biscuits blog for reviews too.
Have an amazing trip!