cost base

I'm not UK tax resident until tax year 23/24. Every December since 2017, some of RSU shares, in US, was vested. Combined I received 3,000 shares. I sold 2,000 shares before becoming UK resident, and sold the rest 1,000 shares during tax year 23/24. Now I'm trying to calculate the capital gain tax. My understanding is that the cost for RSU = the vested price. Since I have 7 vested price (every December since 2017) does the the cost for my rest 1,000 shares to be on "first-in-first-out" (so cost = the vested price of the most recent vested 1,000 shares)?

Hi,
Those shares vested prior to becoming UK resident are not taxable in the UK.  
As the payment is from your employer, the income should be shown in the employment section if it is included in your P60.
You would then claim credit for the Tax in the foreign section under 'Employment, self-employment and other income which you paid foreign tax on'.
If it is not included in your P60, please include it on the box on the employment page for 'Tips and other payments not included on your P60'.  ERSM20193, link below, advises that when RSUs payout at the market value on what is called "dividend equivalents" in either cash or shares, such payments will generally be taxed as earnings in the year they are received.  
ERSM20193 - Employment-related securities and options: what are securities: RSUs and dividend equivalents.
Thank you.