Was I wrongly failed?

I've just failed my driving test and I want to know if I have grounds to complain. Basically, I came up to a roundabout and selected the middle lane to go straight on. Okay. I then went round the roundabout, staying in my lane and exited in the right hand lane. I was in this lane for no more than 30 seconds and in that time, the van behind me started flashing his lights at me. I then moved over into the left hand lane. The DSA Guide says a serious fault (What I got) is for "Significant deviation from the defined outcome with safety, control and/or legal requirement breached" I was In control of the car, There was no safety issue, It's not like me driving in front of this impatient van was unsafe and I can't find any statute which says you must drive on the left at all times always. I think it was unduly harsh to fail me for such a short stint in the right hand lane. Does anyone agree or have I got it wrong? Thanks :)