I’ve had good luck asking rstaurants for burger and salad with lettuce instead of the bun.
Subway can create a salad for you instead of a sub. Or, I’d be OK walking into any fast-food restaurant and asking for a couple burgers and side salad and tossing the buns. (I can’t actually do any fast food places, including Subway, in real life.)
We don’t have the kinds of convenience foods you guys do in the US. I went to a conference in the US last year where I was able to buy pre-boiled eggs, pre-cooked bacon, pre-chopped celery and so on. I was so surprised at how many ready-to-eat things there are - I’d never seen anything like pre-cooked eggs or pre-cut celery here, but it worked really well for a hotel room that only had a fridge. I ate boiled eggs and bacon for breakfast, tuna sandwich for lunch (on low-carb bread I had brought), and salad for dinner every day.
But I’m way more restricted than regular low-carbers because I have multiple food allergies, which makes just grabbing things from random restaurants or picking and choosing from a buffet or menu impossible. If I didn’t have allergies, I think eating low-carb is easy once you get into it, because you don’t have cross-contamination issues to deal with. You could just order a regular menu item and not eat the stuff that’s higher in carbohydrates.