Spouse as Additional Driver - UK?
 

Do I have to do anything to add my spouse as an additional driver for a UK rental? In the US, no action is needed for a spouse.

In two recent UK rentals, the answers I've received were (1) no action needed and (2) no charge if you have a BA FF number, but must see her license.

How does Avis Preferred fit into this?

The norm (across all UK rentals, not just Avis) is that the statutory minimum insurance is provided by the rental company. They must have that in place or the car cannot be registered. A driver is only covered on that policy if named on the relevant rental contract, and if you were stopped by police for any reason I would expect them to check that the driver is named on the rental document (to satisfy themselves that the driver s insured).
Therefore you must have the additional driver listed, even if a benefit (such as BA Executive Club) means you get the extra driver free.
Yes, it slows down pick up as you need to get any pre prepared documents reissued.

Is there any way to speed the process, especially since they've seen her license before?

"Is there any way to speed the process"
Not that I've discovered. I have to go to the desk every time anyway, as Avis persistently add on extra insurances on the contract that I didn't ask for.