I'm a little confused by your 1st paragraph - as you say you stopped to allow traffic to pass before pulling onto the roundabout, and if you only saw the 'speeding' driver when you were at 30 mph, especially as a learner, you must be way down the road already. In my (humble) opinion, even if roundabout looked to be mt, and assuming no obstruction to your right, you shud have spotted a speeding car approaching and waited for him to clear - you may have had a 'minor' for hesitation. T-junctions and RBs are the bane of most learners, as I know well enough (having recently passed..lol), and perhaps your 'planning and observation' needs a little more work.
You will get no satisfaction from Test centre trying to 'revoke' the examiners' decision..don't go down that road.
I had the 'dubious' honour of failing test 3 times, in hindsight because I simply was not ready, and I mostly blame my (then) instructor for making me think I was. I had 'serious' faults for diff'rent things on all tests, so only sensible attitude is to learn from the mistakes..especially as they cost 60quid a throw..lol
By all means,have another coupla lessons including them big RBs and dual carriageways, and address any 'minors' you had..I'm sure you'll nail it next time Lady.. g luck..