I don't think there is such a thing in the UK. I am guessing the equivalent might be Sea Salt?! Or can I just use table salt.
Kosher salt is just coarse table salt (NaCl).
Kosher salt is additive-free salt, generally used for 'koshering' meat according to Jewish practices. Its grains are small, flat crystals and dissolve evenly in a mix
Kitchen Butterfly is correct, and despite what some folks think it doesn't have to be blessed by a rabbi. It is indeed used for "koshering". It's not a sea salt, but a coarse sea salt is a perfectly good (and perhaps better) substitute.
Bacause Kosher salt has such large crystals, be aware that 1 tsp of kosher is about equivalent of 1/2 teaspoon of fine grained salt and adjust your recipe accordingly. Kosher salt does not have additives such as iodine and anti-caking agents like table salt does. Sea salt is a fine substitute.
All (additive-free) salt is kosher. The reference to kosher salt simply means it is coarse salt which can be used for koshering (or kashering) as Kitchen Butterfly said.
I use Kosher salt and sea salt in equal quantities, especially Kosher salt which consists of a hollow crystal.
There is indeed kosher salt that IS actually kosher, even if that's not where its name comes from. (No doubt there are kosher 'style' salts available too - no additives, flat grain, etc. - that aren't technically kosher.)
Look for the little 'U' symbol on the box - even the common supermarket brand, Morton's, has it - if it's there it means it's been produced/handled per kosher law, overseen by a rabbi and so on; it's not really just a matter of what's in it.
Remember that that the two major brands Diamond and Morton's have different weights.
That can really throw off a brine. Most recipes default to using the Diamond Brand unless specified.
Cooky was also right about the NaCl - Sodium chloride, common salt, which implies no additives. Of course, "table salt" depends on what you buy.
The kosher salt they have in the US dissolves really quickly and easily, which can be handy at the end of a dish, rather than the coarse sea salt. I've never seen the same kind of salt outside the US.