Avis Preferred Plus usually requires five Avis rentals in a year and €1,000 of spending. The benefits are not huge:
priority car availability
a free extra driver on every trip (already a free benefit if you book via the BA Avis website)
a complimentary vehicle upgrade whenever possible
voucher for a free European weekend rental after your 3rd rental
You can find out more about the different Avis status levels on their website here.
You can claim your Preferred Plus status by following the link on this page. There is also a notional ‘20% discount’ for Priority Pass members although this may not beat other partner rates such as the BA one.