If it's a small frame & you can get the forks removed then you might be able to get it sent through Australia post.
They allow 105cm length & 140cm girth. So reall that is limited to a 50cm frame with 16cm width & 54cm height
IF it did then AP offer 4 services from Aus to UK, for a 4-5kg package (frame plus packing) that's around AU$145 for Express post, $139 for Airmail & AU$67 for surface (no enhanced insurance available on the latter two & no signature) http://auspost.com.au/apps/international-parcel.html