I work for Royal Mail in a sorting center. The very biggest thing that results in your parcel or letters getting lost or misdirected are post codes. We work from postcodes and if that gets damaged or is badly written then we have to go by another detail, Nottingham for example, and we have to flick through a book that has every postcode for the UK and offshore islands, and doing that is time and money so what generally happens is the sorter will guess, and if it's wrong.. well then it's going to sit at where ever it went until it gets sorted again, and probably wrong again because the address is wrong.
The biggest issue is missing postcodes, as I mentioned it's a very important detail for hasty mail delivery, it's most important for the sorting centers, it's the sorting offices that work from street names. Now I don't know what happened and when but at some point education stopped teaching how to write an addresses properly. You would be shocked and disgusted at how many items we handle without postcodes.
The second biggest issue is neatness. Okay so you went to grammar school or consider yourself posh, studied art and want to be expressive. Stop writing like a ****, because not every RM employee studied hieroglyphics. And I hope you aren't sending to Wales because then it gets reallllllllllllllllllllllllly complicated as every address in Wales seems to have as many L's as I just typed in this sentence.
Yes mistakes will happen that is RM's fault, but other then that I advise you for better safe then sorry to put the senders address on the item twice, or just the postcode, and put a return address on the back. If an item gets wrongly delivered and finds it's way back to RM and there is no return to sender address then it's considered a Dead letter and gets destroyed.