The north London ones are super-selectives. The ratio of applicants to places is 20:1 at HBS, 10:1 at QE Boys and Latymer, and 4.5:1 at St Michaels, but only accepts girls who have taken 1st communion and attend mass weekly(ish). They all have different exams too. Details are here: https://www.elevenplusexams.co.uk/schoo ... ld-11-plus
and generally it's hard to do really well on several types of exam as the time factor is an issue. DD2 sat St Michael's (Maths, English, VR and NVR), and the next day sat Latymer CEM. Quite a challenge for a 10 year old! As a result they do well in the tables, but lots of children at other schools do equally well in smaller numbers. It would be easier to gain a place in Bucks, and there are some great train links to London. The fares wouldn't be much different either.
An other option is to look at state schools who perform well, and their catchment area. Muswell Hill is good (if pricey!), with Fortismere (state school), and the selectives HBS and Latymer an easy journey, as is St M's if catholic, and some great primary schools. Southgate has Ashmole (state school) and a direct bus to Latymer. It's in catchment for DAO too.