Actually yaar, it's very hard to pin-point the exact conception day. Even the NHS midwives will tell you it's usually 2 weeks after your LMP. Just go by the scan date.
In the UK, the dating scan around 12 weeks is the most accurate way they estimate your conception date. Don't worry about trying to calculate the exact intercourse day.
I completely understand how confusing the dating system is, I used to overthink this a lot! The NHS dating scan will clear all your doubts, so try to stay calm.
Save your time and don't bother with online conception calculators. Just rely on the crown-rump length (CRL) measurement from your 12-week scan, that's what doctors trust.