Figures from the International Centre for Prison Studies show England & Wales have a combined prison population of 84,430 inmates. That is the highest in Europe, just beating Poland to the top post. When taken in consideration the rate per 100,000 citizens, England/Wales still have the highest figures in Western Europe, 148, only 4 countries from the East fare worse, Poland (217), Hungary (186), Romania (155), and Czech Republic (154).
Considering that 13% of the detainees are pre-trial, as opposed to Italy for instance with 37%, it seems that there are either a lot more crimes punished with prison term or simply more crimes committed.
Now, foreign crime in any country sparks debate and it influences immigration policies and theories. While some countries clearly do have problems in that area, England/Wales foreign prison population amounts to 13% (73% in Switzerland, 63% in Greece, 27% in Germany).
Official capacity stands at 77,699 so the occupancy level is 108%, which is not as bad as in countries like Italy (136%) or France (117%) but still one of the worst in Europe. As most detainees are there after trial, England/Wales would need to build more prisons to cope with overpopulation.