Those numbers don't quite add up. If your lmp was 12 weeks ago you should be 12 weeks pregnant- even though you would have conceived about 10 weeks ago, so end of August - pregnancy is counted from day one of LMP in the UK. Where does the 10+3 figure come from?
If you have irregular periods the scan date is most accurate by 12 weeks instead of lmp. Your due date will be set by your scan measurements even if it is different from your lmp. I had irregular periods and my scan measurements meant the baby was older than by my lmp date iyswim. It's difficult to pinpoint ovulation anyway because in the right conditions, sperm can live several days awaiting an egg.
If your cycle is irregular the LMP date can be quite a bit off. I missed a period completely before conceiving my son - the LMP date was 4 weeks before the date by scan, and he was still a week overdue by the scan date! If your scan says 10+3 then go with that and you then conceived 8 weeks and 3 days ago.