I think the most important thing is DON'T apply till your monologues are 100% ready. For some places you have to wait ages for an audition, others can be very quick! You have to be very confident with your monologues, as re-direction is common - sometimes quite radically. Also take time to choose your pieces, checking requirements for each college. We had a harrowing experience the night before one audition when DD looked again at info and realised her ist choice piece was not OK as it said "British" play (hers was american). So I suppose another tip is read, read and read again all the info of what the college wants. They are all infuriatingly different! It pays to have more choices than needed (see above!!!). You also need to know about the play, writer etc not just know the piece.
I think it is always good to have someone to work through monologues with - need not be a teacher but helps if they are familiar with drama school auditions. I think it gives you more confidence if you have done this.
Many Acting (not just MT) will also require a song (esp in later rounds) so you need to prepare for this as well.
As for when to apply, I would not apply too early (see first paragraph) but too late and many places may already have been allocated. However, if the college is part of UCAS there is less flexibility.
Hope some of this helps! Good luck

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