The Economist article cited above is quite right in suggesting that the prison population crisis was forseeable. As well as normal population growth, increases in both sentencing rates and length of sentences since before the mid 1990s (even if these have tapered off in the last couple of years) have contributed to the rising prison population.

However, the article provides no justification for its claim that the 15.5% rise in the police force over the last 6 years has anything to do with the filling up of the nation's prisons. While it is certainly true that the British police have become better at recording crime, with the Recorded Crime statistics getting closer to the BCS's estimate (due in part to increased recording of minor offences), there is little available evidence to suggest that the proportion of crimes committed that go to court has increased, and certainly nothing to suggest that 15.5% more of the baddies are getting caught (assuming that this is what the extra police were supposed to be doing.)

As for the suggestion that extra police are tackling more difficult crimes, such as drug-trafficking, the evidence cited (absolute number of prisoners on drug-related offences) is also weak. There may be a correlation between police and trafficer-prisoner numbers, but this does not imply causation. Changes in the number of prisoners in on drug charges could be due to an increase in the drug market (with conviction rates unchanged). Alternatively, as sentences tend to be longer for drug trafficking, the full effects of any change in the number convicted in any one year will take a long time (then length of the sentence) to filter through to prison numbers. It is also possible that the change in the number of drug-related offenders in prison could be due to any other unknown factor.

Increasing the size of the police force may well have been a "Politician's Logic" response to catching criminals (Levitt's study of the US shows that police numbers have very little impact on crime levels themselves, so catching more criminals would seem the only logical reason to increase the size of the force).

And of course, if there are robust data to support the idea that having more police has meant that more criminals get caught, then I would have expected the Economist to have included them!