Don't backpedal OP. You have said you're pretty sure SLT would have authorised your absence anyway but wouldn't authorise your colleague's. Seeing as you have no idea whether there are difficult family circumstances behind her weekend arrangements, that's a weird assumption to make. Also, if you were so sure SLT would have seemed your reason acceptable, why not go through the correct channel and request time off? At the very least, your colleague's favour allowed you to take paid time without it being recorded officially. No school is going to let you have unlimited paid time off, it's down to the goodwill of SLT and may be, for example, one occasion per year. You have managed to gain paid time off without using up SLT's goodwill quota.
Another thing that springs to mind... As this happened in June and was a 6 form class which was covered it would have been a class about to take either AS or A2 exams in your subject which is an extremely important time and there is no way your colleague who shares rhe group would have felt comfortable just sitting doing her own work without any interaction. God knows if I ever cover any class I usually teach, I find myself answering questions, discussing, helping- it's what teachers do. The only other possibility is that it was a Yr 12 group who had completed all their exams and were starting on the A2 component - again, a key time in their learning process.
Do the 3 way time swap which I suggested earlier which can solve this problem (IF a third colleague is comfortable with unofficially covering lessons)
Then in future go through the proper channel when you want to take time off.
Those of us who are teachers know how tricky ANY time off during term time is. Your colleague did you a big favour and it would just be, you know, nice if you could find someone to help repay it. Not because you 'Owe' her but because it's a nice thing to do