More info which puts the incident in context: the lower of the two Warriors was inbound off the 20 procedure. Even though (speculation here) they may have cancelled IFR, they would have been at MDA at approx 2 miles final...at 760' QNH, and would have had a restricted lookout from the left hand seat, since the screens are usually still up or in the process of being taken down at this stage. Add to this left hand circuits on 20 and you can have a difficult time seeing a/c descending on base leg from 1500' QNH. I would add, that TWR at Oxford alert circuit traffic to a/c "inbound 20/09 procedure" as soon as you've called base turn complete. Therefore traffic in the circuit or joining base should be looking for / expecting the instrument traffic on final at low level. The only issue I can raise regarding this, is that during the VFR training there was very little in the way of introduction to the IFR procedures...(in terms of background knowledge), and it's not inconceivable that many of the less experienced students have any idea what ATC are going on about when they report instrument traffic inbound on either of the procedures, unless they are at the stage themselves where they have flown them in the sim.