Under GDPR, Data Breach Reports in UK Have Quadrupled

Good. The point of GDPR was to hold companies more accountable for the data they hold. I'm glad that it's achieving what it set out to do.

Hopefully this will make companies think of data as a liability as well as an asset.

So, previously at least (!) 75% of breaches known to companies went unreported, more likely 90%...

The article does note some over-reporting occurring, so maybe discount that figure somewhat.

When the UK finally Brexits, will the EU's GDPR still apply?

Not counting that the UK might very well have their own version.

All EU law will be directly transcribed into British law, so yes. We could choose to change or revoke it through an act of parliament, though.

Yes.