I think there are 3 things here at play.
1 You have to ge through the bot screening process eg A Level grades and the anticipated or actual degree grade. On another site someone posted % on how many people at Slaughter and May ( where my dad trained back in the day when any degree was good) had 1st and it was the majority. So try to get the best A Levels and year 1 and 2 grades and they do look at individual grades in university exams or call for script eg someone said contract law exam ( which I believe is in year 1 still) might be looked at. The thing is not to party too hard in year 1.
So if you like Politics English History and will get an A star AA in a combination then do that! That person will get though better than someone with AAA in Maths Chemistry and Biology if the firm demands A star AA.
2 There might be various other stages before an interview with a partner who really does not have time to spend on interviews with hundreds of people. At various stages things like extra curricular might come into play if you have got through with a mid 2:1 prediction as at this stage it might help keep you in the game.
3 Finally if all good you get to a partner and can impress with your social skills.
Upshot do the A Levels to get the best grades and do the degree that you will do well in. Really just do stuff you enjoy.
However if you have a skill for Science or Maths you might carve out a niche for yourself in some areas that other people might find complicated.
The only issue with Maths are the people who dislike Maths and dread the accounts exam or worse fail it. So as I keep saying a decent GCSE would help. Obviously when transferring millions you don’t want to mess up and certainly DH always seems to have number scribblings.
Overall I never fully understood who got what training contracts as even in the middle ranking firm I was in there were people with Oxford degrees or 1st from Bristol. (Actually one was in my team that I ran). Sometimes I think that it’s luck or just having a good day. The person in my team was personable enough.
Conversely some people will lesser universities on their CV managed to become senior partners at top 25 firms on over a million a year. Back in the day I spoke frequently ( partied) with such people.
Also some people go to Oxford get a first and never make partner anywhere. I have a friend like that - a very good lawyer but not suited to the cut and thrust of big law.