It is not necessary to have a GP referral to see an osteopath. However osteopaths can and do work with GP’s and other healthcare professionals both thorough the NHS and privately.
Most patients however see osteopaths privately without a referral.
Under the National Institute for Health and Clinical Excellence (NICE) the recommendations for low back pain include utilising manual therapies such as osteopath for treatment. https://www.nice.org.uk/Guidance/Conditions-and-diseases/Musculoskeletal-conditions/Low-back-pain