I have been renting a few times with Avis via BA as I found the rates excellent.

You are covered but there are potentially high excess fees to pay.

The reason I don't pay attention to that is that I always buy the annual rental car insurance, which is a real bargain if you rent a lot. To give you an idea, it's currently around £80 ($120) multi trip, world wide, $0 excess, tires&windshield included cover for 1 year. I can't remember how much car rental companies charge for their add-on insurance, but I am pretty sure that a 2 week rental additional insurance would cost you much more than this annual policy bought from an independent insurer.

In the UK, the insurance industry is heavily regulated and the insurer I use was recommended by consumer rights groups and various online review forums. I don't want to give you the name as I don't want to spam the forum(?) ,PM me if for the name, but you need to be UK based with this one.