Dual Citizenship expired passport querry

Hi all
my 1st post on here, so please bear with me and will be as brief as possible.
Ok so my 17 year old daughter is in Australia, lives with her mum.
She was born and has lived in the UK since 1999, then emigrated to Australia in 2008.
Has an expired UK passport since May 2014 (her 1st UK passport).
Also has a current Australian passport since September 2014.
She now wants to come back to the UK and stay with me indefinitely.

If she travels on the Aus passport, UK immigration will stamp it saying she can only stay
in the UK for 6 months.
My question is, can she enter on the Aus passport and whilst here gets her UK passport renewed then leave the UK so that they have a record of her exiting the country in the the specified time period and
re-entering on her UK passport.
or is there an easier/simpler process that could be followed?
or can she enter on her expired passport being a UK citizen?

Any help would be greatly appreciated with this.
Many thanks in advance.

Kree8