Help & Advice
Deadlines And Penalties - Self-assessment
You will be charged £100 for missing the initial deadline, Further penalties will be applied if your tax return is more than 6 months late.
You will also be charged interest and penalties for late payment.
https://www.gov.uk/estimate-self-assessment-penalties
You can appeal against penalties providing you have a reasonable excuse.
Tax Rates& Thresholds
https://www.gov.uk/income-tax-rates