Find out from the universities directly if a candidate who has retakes is disadvantaged at all because I think you would be surprised thats its more of a myth than the actual truth. (Especially Bristol/Exeter/Manchester)
If you have finished A-levels already and you are now considering retaking etc - then i would take the year out to do retakes then apply with AAB/AAA to Universities. I would try ensure you get AAA prediction as the minimum. Most universities, will give you offer based on AAA predicitons & if it comes down to the fact that you get AAB on results day then there is a good chance you will get in - as I know many candidates this year who missed their Law Offers but still got in. Eg. AAB into Manchester University for LAW.
To go from ABB to AAA isnt that difficult as you only have to retake certain units. Your most important goal is to now have a different approach & see what was ineffective first. Please remember If you apply the same strategy again as first time, you will get the same results, which is known as insanity.
If you do choose to a gap year, then you have just chosen to open hundreds of doors of opportunity - especially a chance for to discover yourself.
As someone, who is a science student in the final year of their degree, I have mentored dozens of students to get into university - the biggest mistake you can make is to rush into a university or course you don't like. You will most likely do only one degree in your life, so make it count & enjoy it because your primary education ends only in 3-4 years & then it stays with you forever.