//We already have one of the largest prison populations in Europe.//
Indeed we do. And the reason for that is because there are far more criminals in the UK than in other comparable countries. Leaving aside those sentenced for very serious offences such as murder, rape, manslaughter, robbery and so on, which attract a prison sentence for a first time offender, the vast majority of people going to prison for the first time in the UK are not first time offenders. They have usually gone through either all or most of the “apprenticeship” of sentencing options available to courts: youth referral orders; discharges; fines; community orders (of various types); and suspended sentences. They end up in custody because all other options have failed to modify their behaviour.
The UK does not commit people to prison at the drop of a hat. In many European countries, if you commit an offence of assault where injury results, even if it is your first offence you will almost certainly receive a custodial sentence. In the UK, if you do so – even if your victim is a police officer – you will almost certainly not. Look at Spain as an example (useful because it sets minimum sentences as well as maximums):
The minimum penalty for an assault in Spain where the person concerned suffers any injury is imprisonment for three months. In the UK offences such as Common Assault and Assault occasioning Actual Bodily Harm will almost certainly result in a non-custodial sentence.
Possession of Class A drugs (heroin, cocaine etc.) carries a minimum penalty of three months in prison. In the UK a custodial sentence would not be considered unless large amounts of drugs were involved or the offender had a lengthy record for similar offences.
In 2018 the EU average prison population per 100,000 people was 111. In England & Wales the figure was about 150. That is certainly the highest in Western Europe. But that is not because the UK is sends people to prison more readily. It is because there are more people in prison who are committed criminals. By the time many people go to prison in the UK they have already committed a large number of offences. They end up in prison because of that, not because of the sentencing regime (which by many comparisons, is quite lax).
The EU average is dragged upwards by the eastern nations. The highest per capita prison populations can be found in Lithuania, Turkey, Estonia, Poland, Albania, Montenegro and Latvia. It is notable that the UK has a high number of residents who originated from many of those countries and among the top five foreign nationals in UK prisons are those from Poland, Albania and Lithuania.
The UK does not send more people to prison because it has a harsh regime. It does so because it has a high preponderance of serious and habitual criminals among its population.