Yes, building prisons is expensive. But the current UK government seems to treat it as a physical impossibility, while releasing violent offenders early due to lack of capacity.
early release is not a problem by itself. honestly the best way to reform the criminal justice system would be replacing fixed prison terms with release based on a psych evaluation and completed volunteer work. of course when you need to release offenders for capacity you should start with the least dangerous ones and i dont think thats happening here, its just random or going off time served.