Current Sevenoaks parent (name changed) and love love love it. Live in Kent, chose it over SS grammar. Seems to manage to get stellar results without feeling like a hothouse. Sense of curiosity and enquiry when it comes to academics, very light touch on assessment in lower years, with more of an emphasis on learning for its own sake, which feels right at that age. Lots of focus on student independence and service, nice feeling of modernity and not being bound by tradition. Lovely campus. Sport, music and drama are very inclusive - lots of opportunities and recognition even for those who aren't 'stars' in their field. New head seems great so far. Very international outlook. Obviously lots of wealth around, but has always felt pretty down to earth - few extras on the bill, a sense of awareness from the school that not everyone is loaded, increasing focus on bursaries.
Re the IB, we're not quite there yet, but from talking to others I think it's not necessarily harder than A levels but it's not for everyone. Suits a relative all-rounder, and perhaps a greater volume of work than three A levels? There's always the option to move for sixth form - plenty join at Year 12 but some leave too, to do A levels elsewhere.
Be aware though, that although Sevenoaks has plenty of local kids, it also has quite a lot who travel out of London on the train (plus international boarders of course), so don't assume that all the day kids will be local friends. And don't forget to look at transport - if you're not walking distance, then there's no school transport apart from the minibus from the station.
Don't know Caterham at all, so can't compare.