100% percent worth it for me. I strongly recommend it
Allowed me to be much more financially secure at uni. Also got me to get used to managing my own time without school telling you where to be and when 5 days a week.
I don't think it made me any more behind my peers, but if you're worried about that you could chose to self study an additional module/a-level during the gap year. You don't have to take exams in it, just keep the brain busy. I self studied a further maths module my school didn't cover in mine.
I applied deferred (so I applied saying I'm going to have a gap year this is for two years time), so I had a secure offer during my gap year and wasn't worrying about that, but it also allows you to change your mind if you realise during year 13 that you were wrong about what you thought you wanted to do.
Hope that helps