You cannot leave the UK while your visa is processing.
As your visa has expired, you are now on 3C Leave, which allows you to REMAIN in the UK legally (and continue to work) while your visa is processing.
However, as soon as you leave the UK, your 3C Leave becomes invalid... which means you will lose your right to reside in the UK, and you will not be allowed back in again, as you will have no valid visa allowing you to live and work in the UK.