I think I heard that the Illegal immigrants that are in our prisons is around 10% and then there is the masses of young offenders, and repeat offenders for miner crimes.
Also prison jailers have are now at their lowest numbers on record. They'll be either privatising it or enrolling the army to deal with it soon.
Yes building more prisons and freeing the petty criminals is one solution. I suggest a shake up in the prison system entirely. From segregating criminals based on their crimes, for example all paedos on high sentence in one place, violent murderers in another and carer burglars and robbers in another and so on. So the lesser criminals would be held in another prison entirely with internal rehabilitation measures to teach trades, and other socially equipping schemes. The segregation will shield them from the mainstream offenders so consciously they are still separate and not yet apart of them, and they cant learn experienced tricks while inside.
Longer scentances for the real criminals, shorter but totally constructive internments for the 'beginners' to give them opportunities to redeem their lives and no imprisonment for very miner crimes like cannabis usage or not paying TV licence- comunity service, fines and other restrictions in the comunity like not being allowed to leave the country or go to the pub
or taking away their driving licence etc could be used instead.