The best thing to do with sightreading is to just keep going & don't correct yourself when you make mistakes. I find it extremely hard not to correct my mistakes as I'm such a perfectionist but if you do then you mess up the rhythm so it's best if you just move on. The only other advice I can give you (for your next exam obviously!) is to try to read ahead if you can as then you know what's coming.
The sightreading is only one part of the exam & considering you did so brilliantly in the other parts I'm sure you've done really well. People get so nervous when they've done an exam & every mistake seems so much worse than it actually is. Remember the examiner is looking at things as a whole so they won't be knocking off marks for every mistake you make.
When I did my grade 4, the rhythm was really tricky & even though I didn't get it quite right I got all of the notes right & I got 17/21 which wasn't too bad.
Good luck with your results.
Lisa xx