Some of the internal scenes of Oxford University (kitchen, laundry, classroom etc) were actually filmed in the beautiful Woodchester Mansion, an unfinished Victorian Gothic house which is in Nympsfield, Gloucestershire.
The initial scenes before the opening title sequence lasted 8 minutes 30 seconds.
Ian Gelder (Charles) and Lucian Msamati (John Faa) both starred in another HBO series Game of Thrones (2011) as Kevaan Lannister and Salladhor Saan respectively. They never shared any scenes in GOT.
This isn't the first time James McAvoy and Anne-Marie Duff have starred in the same series. They both starred in Shameless (2004), sharing many scenes together as Steve McBride and Fiona Gallagher.
In our universe, the River Cherwell in Oxford is pronounced "Charwell"; in this universe it seems to be pronounced as spelled (36:33).