All independent schools have different entry criteria, but they all have entrance exams which take place in the spring, for September entry.
You would need to ask if your daughter can be considered outside of this time frame.
As you are looking for an assisted place, you would need to check that bursaries and scholarships are available, and how much financial assistance can be offered.
Regarding scholarships - one of my relatives has a scholarship in a top school, but it's more the honour of it, than much financial benefit. I believe it's worth about £20 a term, which doesn't make much difference when the fees are almost £20,000 a year.
There will of course be schools which offer more valuable scholarships but in reality you will be looking for a decent bursary.
To get an interview, a decent mark in the entrance exams is obligatory, and as long as your daughter is neatly dressed, the actual clothes won't make much difference.
Good luck with the process and I hope your daughter can get a place.