I find London to be a decent place to eat without dairy, gluten, etc… There are a number of chains that might work for you—one is called Planet Organic. And there are more! There is a Whole Foods at Picadilly Circus. Google “organic and vegetarian and/or vegan restaurants London, Cambridge, or Oxford” and you’ll find a lot of choices. Also, goodgle “gastropubs”, and if you’re eating chicken, you should be able to find some fairly “clean” chicken to eat at the gastropubs along with a salad or plain veggies.
I travel for work quite a bit. I always bring buckwheat “crepes” with me to smash an avocado into. I also find a salad dressing I can buy where I am traveling, an carry that, so I can order a salad anywhere. Like you, I buy my kefir, have hot cereal in the mornings (I eat creamy buckwheat). I soak my cereal overnight with a bit of lemon juice or yogurt, so that it cooks in the morning in minutes! I also often buy a whole roasted chicken—organic if I can find it, and eat that throughout the week either in a salad or a crepe. Sometimes (after reading the ingredients) I buy a container of guacamole and use that as a salad dressing with my salads. I can’t eat eggs, but I can only imagine how hand boiled eggs would be.
And I always get a refrigerator!
You’ll do great!