Yes --- with 1500 applicants it had to be Wallington Girls. That place is nuts.
My dd was in a prep last year, 4 girls wrote for Wallington Girls, (not DD as she is dyslexic) NONE of them got in. These girls were offered scholarships at Croydon High and Caterham but did not get into Wallington Girls. The one girl was top of her class, classmates got into Whitgift, Trinity, Wilsons, Worth, Caterham, Lingfield, Woldingham, Reigate Grammer and there were around 20 scholarships (18 kids). But no-one got into Wallington Girls.
I think that is probably one of the hardest schools to get into and I would think that you would need to do A LOT of coaching to get it. I think the majority of kids that get into Wallington are seriously tutored to get them there and unless your child is exceptional it is going to be a lot of work.
The tests are Verbal Reasoning and Maths. I would expect, based on what I saw, that you won't get into Wallington girls unless you are on or very close to Level 5 at the end of year 4.
Just my opinion though....