Do you have to take the CDW or does your own car insurance in the US cover it along with your credit card? Do you need to bring the credit card info book detailing that they cover it with you and will the car rental agency accept that? Ive rented plenty of cars in the US and always decline the additional insurance since my insurance on my car is full coverage and that along with my credit card is enough
I rented a car for 3 days from Thrifty in 2013 and declined the expensive insurance. Also, I just booked our rental from Heathrow for our four weeks in Britain this Fall and declined the insurance. I have American Express and used it to book, and will opt for their insurance. It is a lot cheaper than paying the rental agency.
Some countries, like Australia, require you to buy the extra insurance, but not the UK.
If you book online as a USA resident it should be easy to decline other 'insurances'. It is more difficult as a UK resident as very few people have coverage for hire cars - only top end credit cards offer it and regular insurance for your own car doesn't cover it. So UK residents deals are usually all-in; but can sometimes be cheaper than those without insurance offered to non-UK residents.
Look into what your Credit Card actually covers and what you need to do to file a claim. Many people rarely check, then find they are essentially paying any damages, then fighting the insurer for a refund, lacking forms or a Police report, etc.
Some Credit cards, like AMEX, offer premium coverage for a fee (AMEX is less than thirty Dollars for the length of the rental) that covers more, eases the claim process, and is worth the money for international travel, so you might check into your card.
Other than that, keep in mind that you will be driving in a country with truly foreign traffic flow and rules, narrow roads, and multi-lane roundabouts, so your risk for a fender bender is greater, having a bit more than credit card coverage might be wise, or at least offer peace of mind.
I have used AMEX and for one to three day rentals have bought full CDW, zero deductible, from the rental agency. Just do not want to ruin a trip dealing with a claim.
I wanted to stress that reading the actual coverage and procedures listed by your credit card is very important. You need to know this prior to renting your car. This is a very specific question and you can only get general answers on this forum.
On a slightly different note, but insurance could come into play, are you sure you want to drive the same day you arrive? Jet lag, lack of sleep, and not being familiar with the local road system around Heathrow can all be a problem -- not to mention driving on the left side of the road. There are some people that are not bothered by any of those things. Others think they are ok but are really driving somewhat impaired. Still others are complete zombies and should never drive the same day. Perhaps you could take a train to your first location and rent a car there. Don't mean to rain on your parade, just something to think about.
For CDW, I get a separate policy through Allianz for $9 a day. It, along with the CC coverage should be enough. Remember that your CC is CDW only, not liability or personal injury/property damage, so if you hit another vehicle or damage property you're not covered for the damage to the other car or property; only your rental car. Consider getting those through the rental company. As far as driving after the flight, only you know how long you can safely drive after a flight. Some shouldn't get behind the wheel at all. I know that I'm good for about 4 hours, but had better be parked after that.