Hi Lottie,
I am assuming from your description that you have a raised bed, then a path area and the this 50cm strip.
I would not plant Comfrey unless you plan on the Bocking 14 variety. Comfrey is very invasive. You could put Herbs in or various fast growing Veggies i.e., Radish.
But I think companion flowers/Plants for pollinators, Cosmos, Marigolds, Dill, etc., is an excellent idea. I strongly believe every allotment, whatever it’s size, should have such an area incorporated into the site.
Don.