I'm a big believe in the KISS rule when dining, I'm not one of those folks who lives to eat. A tasty simple meal is always going to work for me, and I would suggest that for at least one meal a day you just stop, eat, and go. If you've never heard of them, England and especially London is absolutely chock full of outlets of the chain Pret a Manger (usually just called Pret). They are like a Starbucks in their focus on coffee but also have a wide selection of premade sandwiches and salads (and of course pastries). There will be times where you're standing in the doorway of a Pret and you look to your left and your right and see two more; they are really that omnipresent.
It's an easy meal. Just browse the selections, take them to counter and pay. One point - most British sandwiches will have either butter or mayo, I don't mind the former but really don't like the latter. So read those labels. Salads also contain an ingredient called rocket, which I thought was a joke but it's a type of leafy green.
Another option is getting food to go from the many food court operations. Every department store will have an elaborate food setup, like Harrods, Selfridges and Marks and Spencer. M&S also has standalone outlets, and stores like Tesco will have small operations like a 7-11. Unfortunately, these combo deli/grocery stores have a wide selection that is only for takeaway - no area with tables and chairs. So this is an option to eat in a park or in your room.
You'll also see chains you already know, not as many Mickey D's as I expected but a ton of Subway, Five Guys and KFC outlets. And in a pinch just pop into a good-sized train station and you'll have lots of choices.
Finally if you're like Ted Lasso and can't stand tea, no worries. Coffee is the hot beverage of choice in the UK and you can find it everywhere in all the various and sundry variations you'd expect at a Starbucks.