Reddit mentions of Chef Aid Doughnut Kettle Descaler

Looks like limescale. Welcome to a hard water area. The descaler balls work pretty well, I also use Oust every so often which is pretty nasty stuff but does the job. Filtering your water first also works pretty well if you can be arsed. Whatever you do, never ever drink the last gulp of tea/coffee.

While you're here, consider getting a magnaloo to stop build up your toilet and grab a decent descaler like Viakal to clean your taps/sinks/bath. Also make sure your dishwasher (if you have one) is on a hard water setting (if it has one) to prevent leaving water marks.

You can get these that will help the limescale build up over time. If you are going to get one i would advise getting one before it builds up as it's more of a preventative thing that a curing thing.