Not too far from you, Paul Hill at Corinium Rifle Range:
http://www.coriniumrange.com/
There was someone I think from Oxford way on the last course Paul took. I recently did my DSC1 with Paul who uses BASC assessors (BASC affiliated course) and have to say it was excellent. Despite 30 years of shooting experienced, I went with an open mind and did come away having learnt a reasonable amount. No shortcut to experience, but worthwhile imho and highly recommended. Paul will I think be running the next one in a few months time. His contact details are on his website. He also has the advantage of having his own full bore range close to where the class room stuff is done at Cirencester Rural Innovations centre. Feel free to get in touch if you want an informal chat.