i think only take a gap year if you have things planned. I'm on one currently- and I was literally so lost at the beginning, I had like no idea what to do with my life. and it can be lonely especially if all your friends are at uni. However, gap years do give you a break and a chance to grow as a person, and time to travel if you wish.
And also don't underestimate yourself. I literally thought I would get Cs last year- and I actually done alright. I'm currently resitting because I know I can push myself to do better- but studying on a gap year is hard. Esp when you're not in class and don't have teachers pestering you and giving you exam tips.
If you have good offers this year from unis, I would say take it. I literally got offers from Kings and UCL last year but got rejected this year- so your place is not always guaranteed on gap years.