the only successful prison break using a helicopter in the UK was more than 20 years ago when two inmates were sprung from Gartree Prison, in Leicestershire.
and since then all the open areas at high security prisons have had wires over them. Thus i feel their master plan was fatally floored.!
in fact if you check out live search maps for the prison, you can see the supporting posts for the wires across the sports field..!!