2017 tax year
You will not have to file any US returns for the 2017 tax year. You should file US taxes in April 2019 for all worldwide income earned in 2018. This includes any income you earned January-April 2018 while you lived in the UK. You will be able to claim a tax credit for the amount of tax you paid on this 2018 UK income on your US return to avoid be taxed twice.
For any UK income in future years (i.e. interest, dividends, or capital gains and rental income from real estate), you must pay UK tax. Again, this income should be reported on your US return and then you may take the credit for the amount of tax paid.
Be sure to fill out form P85 and send it to HMRC before you depart the UK. This will let them know that you are leaving the country. Additionally do not forget to include your income from 5 April 2018-3 May 2018, as it must be include on your 2018-2019 return.
When filing your US return in 2019, you automatically get a 2 month extension until 15 June to allow time to gather any UK tax documents you may need. If this is not enough time, make sure to file for an another extension before 15 June. This will give you until 15 October to submit your US tax return.
In California, you will owe state tax in addition to federal (US) tax. This tax is quite high, with the maximum rate being 12.3% for the top income bracket. However, you may deduct this tax payment from your gross income on your federal return.
-Decide if you want to itemize your deductions (figure out which expenses during the year are deductible from gross income) or take the standard deduction (a flat, simple reduction of GI based on filing status). Be sure to do this at both the federal and state levels and figure out which option will leave you with the lowest adjusted gross income (AGI).