No, Keir Starmer has not used the phrase “far left terrorists” for far-left extremists.
He described Jo Cox’s killer as a “far-right terrorist” and has repeatedly condemned far-right thuggery and extremism. He has referenced “far-left extremism” in broader contexts (e.g. threats to Jewish communities alongside Islamist and far-right extremism), but not the specific “terrorists” framing applied symmetrically to the far left.