Be careful with the out-and-back. There was a thread about someone who returned the car early and it caused the car to refare -- suddenly the BAHolidays voucher didn't cover the full amount!

I couldn't find the BAHolidays thread but here was a similar one where it was ultimately corrected.
https://www.flyertalk.com/forum/avis...-refunded.html

And another post that says it can cause a refare: https://www.flyertalk.com/forum/29549680-post2.html

Edit: Apparently it can be done without issue as well: https://www.flyertalk.com/forum/brit...car-early.html