Only half-a-dozen or so rentals, but I've never had trouble getting Sirius activated although I did once lose it half way through a rental. The subscription had run out, so it took three phone calls & a few hours for it to be reactivated.

It's annoying having to ask at the desk for Sirius to be activated - negates one of the advantages of Preferred membership. Maybe one day Avis UK will find out what Sirius is & add it to their booking process. It can be done on Hertz UK's website, but for my last trip they came up with something like £120 for a 20-day rental!