More immigrants. (Immigrants are less likely to commit crime than the indigenous population.).
This is in fact the opposite of what actually occurs. Foreign nationals make up about one seventh of the UK prison population which is far greater than their percentage in the population. That doesn't even take into account those immigrants who are now UK citizens. In some European countries foreign nationals make up an even higher percentage of the prison population. So unless our justice system is racist on a scale that makes Apartheid South Africa look like paradise his claim cannot be true.
What I suspect Andrew Rawnsley has done is either import a meme from the USA (because in the States immigrants do have a lower crime rate than the population at large.) or he has swallowed misleading propaganda from the likes of ACPO.