I've opened someone's parcel by mistake, will I legally get in trouble?

So I messed up the other day, I've been ordering lots of things from etsy, ebay etc where people tend to write their own labels instead of printing them. I got one of those RM cards put through my door saying my parcel has been left in a safe space (beside my bin in the garden) as delivery had been attempted twice already. It wasn't a signed for parcel or anything. I got the parcel and brought it into my house, it had my address on it but someone else's name. I figured it was just a mistake on the sellers end but when I opened it it wasn't my parcel but a Christmas present that was sent either really late or got lost in the system. Only reason I know its a Xmas present is because the parcel inside was bubble wrapped and said 'Merry Xmas love aunt x' luckily it seems like the bag had been reused before and on the inside was the address it was actually meant for which is in the same area as mine, only difference was a 0 (example I'm number 6 and it was meant to go to number 60) a mistake on the senders part. I can't send it back since I've opened it, I would like to take it myself to their door so the intended person actually gets the parcel. But I also know its illegal to open someone else's mail. I'm hoping if I explain myself to them that it was a honest mistake cause my address was on there that it'll be okay, but if worst comes to worst and they're not happy I've opened it and decided to open up a case will I get in trouble? Sorry this post isn't formatted very well I'm using reddit mobile. Thanks for the help everyone.