Don't invest unless you're prepared to lose money. This is a high-risk investment. You may not be able to access your money easily and are unlikely to be protected if something goes wrong. Take 2 mins to learn more.

Is the interest that I earn net of tax?

No. The interest you earn is gross of tax. You are responsible for your own tax affairs. HMRC has published a guidance that you can find here https://www.gov.uk/guidance/peer-to-peer-lending