The IB is good for all rounders and people not sure what they want to study later. If you are a specialist (or in my case, simply innumerate) you might prefer to study fewer chosen subjects in greater depth. Lots of youngsters would hate to do a language or maths after 16 (whether capable or not)
IB is administrated from its centre No govt interference so no political stuff and worries about grade inflation, needing to show standards are rising year on year etc, which is a great advantage.
A Levels are more content driven and structured. Content is clear and quite deep. Ib is less specific in its content - more room for choosing what you study and how, more independent. This is great for self-starters and independent learners. Can be tough though. The learning styles are slightly different, although, if you can teach one you can teach the other and all the extra oudoor/community stuff is compulsory whereas at A Level it is not.
IB is given marks so you can distinguish between someone with 38 marks and 40 marks whereas it is harder to distinguish at A Level though that should change with A*s and Cambridge (maybe others) look at marks gained at AS now. for this reason it can be easier for admisssions tuors
The media adores IB and gives A Levels a kicking - not completely fair IMO.
Pre-U was devised by Cambridge to solve problems with coursework and to differentiate better than A Levels were doing but are still new - first cohort just taken them. Had no idea as many as 100 schools were doing them. The schools mentioned above do a pick 'n mix of A levels and Pre-U, depending upon the syllabus the teachers prefer. I don't think any school considered less than elite is doing them but might be wrong.