I tend to rent from Avis via their BA Avis website
http://www.avisba.com/

Most of my rentals are for 2 days Fri night to Sunday night and I always get avios. Most of the time 500avios.

Though last week I rented in Granada, i did the booking via this website but then at airport i used an avis voucher for a free rental so I didn't pay anything. Yesterday I got 750avios for the rental!

Also you get free additional driver which we always find useful.