All of the suggestions that sea salt, course, fine or flaked, are the equivalent of kosher salt are misleading at best. Kosher salt in the US is a standard kitchen salt, not used solely for koshering. Its larger granules allow for more precise salting of foods during prep, cooking and serving. There are two main brands, Diamond Crystal and Morton's, similar but with different densities so salt is usually used by weight or taste. Chefs and cooks generally use one or the other (I am a Diamond Crystal person myself). The grain size and flowability of table salt make it difficult to distribute or control.
I have not yet found a UK substitute for Kosher salt in cooking. Maldon is great salt but the variety of crystal sizes makes for difficult precision and repetition and using flaked sea salt for salting pasta water say, is a but over the top in the expense category.
The prices on say Amazon are extremely expensive compared to any grocery store in the states. A three pound box of either Morton's or DC will be $3 US or so. Once you start using Kosher style salt or its equivalent, you can save the fancy salts and grinders for the final salting or the table.
The short answer is I have not found a local source in the UK for the two kosher salts in general kitchen (both home and restaurant) use in the US, specifically, Morton's and Diamond Crystal.