Why not just go to one of the major companies. Just did a random check on a 1 day hire from Nottingham to Heathrow on Herz

Including CDW (GBP600 excess iirc), TP and taxes , the cost for 1 days rental for a group C car is GBP32 ( AUD50 )

Using the QFF CDP number ( 297302 ) , the total cost drops to GBP30.41 ( AUD47.30 )

Dave