Is this some kind of "blame it on the Tories" thing?
Is that really all you've got? It's Starmer's policy to release these people early. As stated he could easily free up spaces by releasing and deporting non UK citizens in the prison system.
He could also release people jailed for hurty words on social media posts.
Longer term he could stop sending billions in aid to foreign countries and build more prisons. Or stop illegals entering the country, some of whom commit crimes and end up in jail. That would have the added benefit of easing pressure on the NHS, schools, housing, public services etc. He'd have enough left to reverse his policy of removing pensioners winter fuel allowance.