Estimate of when I conceived?

[KeenMoose]

if I had a positive pregnancy test on the 18th July … is it possible it could have been from sex on the 12th July?

I'd say unlikely, it's just 6 days. Was the line dark or faint?

Around 9 or 10 days later you may start getting faint positives

I wouldn't say so. Would be before that as it takes more than 6 days for egg to fertilise, travel through the tubes and implant. X