(Reuters) - The head of a G4S-run immigration unit in Britain at the center of allegations of bullying and abuse, has resigned, the outsourcing company said on Friday.  G4S said Ben Saunders, director of the immigration removal center near Gatwick Airport in southeast England, would be replaced on an interim basis by Lee Hanford from Monday, Sept. 25, pending the appointment of a permanent successor.  British lawmakers accused G4S last week of repeated failures of detainee welfare safeguards and whistleblowing mechanisms at the immigration center, after physical and verbal abuse by guards was revealed in a BBC documentary. G4S, contracted by the British government to run the Brook House center, earlier this month suspended 10 members of staff, three of whom have now been sacked, for abusive behavior following the documentary. Police are also investigating. Earlier this year an investigation at the G4S-run Medway Secure Training Centre resulted in allegations of abuse and mistreatment of youngsters. The center is now run by the government s National Offender Management Service. 