As an aside, Gromit (and since you raised it) the reason the UK has a higher prison population than similar countries elsewhere is, quite simply, because it has more criminals.
The UK has, in fact, a fairly low rate of imprisonment (prison sentences per crime committed) but one of the highest levels of imprisonment (as a percentage of the population). One of the main reasons for this is that we have the top rate of violence and theft offences in the developed world (though, occasionally second to Australia).
For example, Spain has about half the level (per head) of this type of crime than us and fewer prisoners, but a much higher rate of imprisonment, with almost all violence and theft offences leading to custody.
The fact that the prison population has doubled in recent years is testament to the fact that more crimes are being committed (or at least, prosecuted), not that sentences are harsher. On the contrary, since 2003, a large number of less serious offences which previously attracted custody as a �starting point� have been downgraded to a community-based penalty.
To take the trivial offence of domestic burglary as an example, before 2003 magistrates were urged to send all those convicted of the offence to Crown Court, where the judges� guidelines for a first time offender was 12-18 months. Now magistrates� guidelines suggest a custodial sentence should not be considered unless there are serious aggravating features present. Victims of daytime domestic burglaries where the goods taken are of low value can therefore now expect the perpetrator to serve his sentence "in the community", whereas six years ago he had a very good chance of going to prison.
As you say, best not to get involved in a protracted discussion about crime and sentencing statistics.