We went on this tour on a Saturday, so we went to Warner studios first and then we went to Oxford in the afternoon.
It was a medium size group, I think we were around 15 people. A tour like this one is good if you have a tight schedule because you can see both locations in one day, but if I had the chance I'd rather go by myself so I can spend more time in Warner studios.
Once we got there, the guide gave us the ticket but still we had to join a long cue to get in (it is quick, but still needs time). The visit to Warner studios is free-range, the guide doesn't get in. Still you need a minimum of 3.5 hours. We stayed in the shop (amazing shop) for almost one hour, so we had to rush a little bit for the visit (if you take the audio-guides and you stop at every point, you will not have time for eating or shopping then). The studio visit was great! It is REALLY WORTH IT. There is a cafeteria where we could have a drink and something to eat but after the cafeteria you still have quite a long way to go before it ends.
Then we had around one more hour drive to Oxford. Once in Oxford, we had a walking tour where the guide explained some history and some anecdotes, but the filming locations of Harry Potter are indoors, so you can only go in during your free time. We had like 2.5 hours free in Oxford, which was enough for us because we did not go inside any of the buildings. However, Oxford is a beautiful town and it is worth a visit. The driving was safe and both the drived and the guide were very kind. We left London at 9am and arrived back at around 20pm. It is a long day.