Lovely that your DD is thinking of accompanying - I've enjoyed every minute of the collaborative playing I have done!
Re auditions. Most if not all the conservatories will require a Chopin étude. From either Op. 10 or Op. 25, not the posthumous ones. Also, you don't say which diploma? If it is DipABRSM then I'm afraid these would be considered quite 'easy', although if she demonstrated a really solid technique and excellent musicality, then they might be ok. The toughest conservatories for entrance in piano are RAM and RCM, as competition is high for being in London! The others are excellent though, and had you thought of Trinity, the Welsh Conservatoire, or Leeds College? Also excellent courses, and perhaps a little less pressured. Most conservatories also offer a chamber music option for the last two years.
Having said all that, I think there is also something to be said for an undergraduate degree in a university (and I say this as a pianist working ina conservatoire). There are many that have a good performance element, offer an all-round music education that is particularly useful for an accompanying career. They would also be three years rather than four, leaving a year to go and do a Masters' in accompanying at a conservatoire. Somewhere like the RAM or the RCS do truly excellent postgrad courses that not only set students up musically, but also professionally. I would suggest you look at places like Southampton, Guildford, Nottingham. These places can have more performing opportunities, surprisingly. There are many postgrad accompanists in the conservatories who did not do a conservatoire undergrad degree!