Don’t do it!!
Either go to uni and make the most of it or take a gap year and resit. One year in the grand scheme of things is nothing!
When my daughter started uni there were 7 students in her flat. 6 had done gap years, so you won’t be an oddity!
If you try to do both, even if you manage the work load, you will definitely miss out on the fun of a first year at uni, and that’s something you can’t really re discover even if you change schools and degree subjects. You should be making the most of each moment, and you won’t be able to if you spread yourself too thin.
Secondly, as someone has mentioned, your exam dates could quite likely clash, and then what? A wasted year of uni with an additional debt of between 10 and 20K to pay off or retakes in the summer.
There is no reason to rush into things. If the choices available to you don’t appeal, then take a gap year. Retake or don’t, but reapply with the grades you have and choose schools and degree subjects that appeal to you, and then make some aspirational choices if you manage to do better if ever you decide to retake.