Agreeing with some of the PP's here re not getting too bogged down on A level B/C or that mainly those attending Oxbridge will gain success on the grad internships/FT roles. A level MFL is deemed hard for non native takers, the grade requirements for unis excluding top tier will be lower to attract the candidates, and often in clearing when not filled. Teaching MFL at UG varies so much by uni - many will find the language aspect less intense as there's so much more to cover; it's important to review the language coverage in the handbook if YP wants to come away with high proficiency. However this can always be supplemented with extra curricula and travel. Often programmes can be modulated to suit their interests.
The question the OP asked was where; many talk about league tables but again, this is so subjective so whilst it's best for your YP to aim as high as they can, matching a MFL based programme to what they enjoy and can get the best value from, including their own specific uni criteria needs is preferable. On my DD's for example, being located near a high volume international route network was top of her wish list, for her to progress her linguistic skills.
The ranking variance is vast as the links show.
https://www.topuniversities.com/university-subject-rankings/modern-languages?page=0
https://www.thecompleteuniversityguide.co.uk/courses/search/undergraduate/modern-languages
https://www.thecompleteuniversityguide.co.uk/courses/search/undergraduate/modern-languages
Is it worth it - anecdotally in our opinion, yes. Apart from the frequent parent who says what's the point when most of the world speaks English, the shortlist experience has been similar Norauraborealis' DC (love it, I missed it too) except mine is not at Oxbridge, nor studying in UK.
Given that there is so much free and accessible support on recruitment processes, materials etc for many sectors, ensure your YP values the need to retain maths skills to undertake non-law assessments, even if pure humanities biased. This, on top of leading as full a life as they can with societies, PT jobs, basically anything that can help their application have some standout, as well as getting them to articulate succinctly if they get through to interviews (AI or live), using STAR or whatever methodology that works for them. They'll often get an immediate summary of their psychometric profile if they've got through initial selection, this can help them identify their gaps - other candidates will have them too from top tier unis, such a leveller if these basics are not addressed.
There is never a better time to get over the top roles going to others as more recruitment blind, diversity, WP initiatives etc are extended to reflect within the workplace more. The support is out there for self-driven YP, whatever they are studying. Including MFLs so good luck to all those thinking about it!