Be aware of the following:

1. CDW/LDW in the US works differently than in Europe. If you buy it in the US, it covers damage down the last penny. In Europe, often no matter how much coverage you buy, you still have sizeable deductible that they can bill you for afterwards if they find any damage.

I'd be concerned whether the UK site is adding European-style coverage, while the US isn't adding any automatically, but if you add it manually (either now or at the rental counter) it'll actually cover you better.

2. Yes, you can add these coverages at pickup. Somewhere on avis.com it should tell you what they would cost, but it may take some rummaging around to figure it out, depending on how you are making the reservation. (If you are making the reservation without logging in, it may show them as options for you to buy; if you are making the reservation while logged in and you Wizard account is set to decline everything, it may show "declined" in the space where the price would be. So one trick would simply to go through a dummy reservation process while not logged in to get those prices.)

Having said all that, the CDW/LDW prices in the US can be very high, or at least seem espcially that way when coupled with daily rates that are (in some cases) very low. (In the US, if you take this insurance, it can often double the overall rental cost.) That's why in the US people often decline this, using either their own personal auto insurance or credit card insurance (on the chance that they'll be hit with one of those "delightful bills").