I am from the UK and did A-levels, went to university in the UK. I currently teach in an IB school in the EU.
If my DC were going to study in the UK I would go for A-levels. I personally loved being able to pick my subjects (drop Maths and Sciences and focus on Arts, Language, Humanities). This worked well for me when picking a uni course in the UK as my interests and strengths complimented each other.
I like the IB courses but it's very challenging for students who are more Arts orientated as they can only ever take 1 Arts subject in Group 6 and have to continue with Maths/Sciences.
I think the IB is actually more geared towards those who are more STEM orientated as it's easy to take HL Maths and 2 sciences, with your SL being first and second language (or 2 first languages if you're bilingual as many IB students at International schools are) plus your Humanity (Geography is favoured among more Science students and Economics requires good Maths skills). Or students can request non-regular with 3 sciences and drop their humanity if possible (I think this is only possible if they can prove their university course would only accept them with all 3 sciences - medicine or veterinary in my country).
I do think if intending to study abroad then IB is better (the UK is the only country I know of where students can drop core subjects like Maths, English, Science in pre-university courses). See my earlier post about university admissions in the Netherlands. They say they would not accept Music or Drama A-levels as equivalent to the Dutch high school diploma, whereas in IB students could take an Arts subject in Group 6 if they wish and meet the requirements with overall points.
I think you really need to look at what your child's strengths are as well as their future plans.
I would also consider the social implication of moving my child from an IB only school to an A-level setting. Will they have to start again socially?