casual foodie visits london
If you are around on Thursday or Friday lunchtime, try the food stalls at Whitecross Market which is right near Barbican.
Exmouth Market, also not far from Barbican, is also a great street both during the day when there are food stalls or in the evening. There are a number of small restaurants there where you are perfectly fine going alone. If you like Spanish food, Moro is fairly pricy but amazing, but their tapas bar Morito next door is cheaper and more casual. Caravan is a gastro pub that is great for drinks or coffee, small plates, big meals and Sunday brunch - the menu runs the gamut from wontons to couscous to Weinerschnitzel - and I've never had a bad meal.
If you like Thai food, there's a very authentic Isarn-style place in the basement of the Heron pub on Norfolk Crescent off Sussex Gardens. There's no sign and the crowd is mainly Thai, especially on weekends when they have Thai karaoke. But during the week it's quieter and no problem going on your own. They have a whole page of som tom (green papaya) dishes. They don't dumb down the dishes for foreigners unless you ask for it mild.
Lots of good stuff in clerkenwell/farringdon near Barbican. You're striking distance from St John, which is maybe a bit overpriced but justifiably iconic.
Vinoteca in farringdon would be a good place to get a light dinner and a glass of wine.
Kateh offers nice persian food a short walk from Paddington.
I doubt you'll feel out of place at any of those options.