I'll just throw out a couple of thoughts.

First,

August... Death Valley...

Please don't tell me next that you want to rent a convertible Mustang for this drive.

Want to know why it's called Death Valley? Want to take a guess?

Look, I love the UK and lived there for a number of years, but I simply do not get the British infatuation with visiting Death Valley in the summer. It will be hot. I mean, harmful, dangerous hot. I mean can't get-out-of-the-car hot. Seriously. Belay the thought my friend. Visit DV in January or March and you'll be able to enjoy the many fabulous things to see and do there.

Second,

August... Yosemite...

If you don't have accommodation booked right now, good luck with that. You will be joined by thousands and thousands of your new friends queuing up to see waterfalls which, given the current ongoing drought in California, will be trickles at best, but (most) will probably be dry. Come in May, different story. And if next year is (God forbid) anything like this one, a visit to Yosemite might help you familiarize yourselves with forest fire fighting and the detours and awful conditions that accompany fires.

My recommendation instead: https://goo.gl/maps/v4SaWuoxCWK2

Start in Vegas, which is an excellent place to let your body clocks adjust to the 8-hour time change. When the clock says 4 AM but your body says noon, no worries, things are happening downstairs or out on the Strip anyway.

Do the Grand Canyon, then instead of backtracking to Death Valley, head for Joshua Tree National Park and Palm Springs instead. Joshua Tree is spectacular, and while it too will be seriously hot, it's frankly a national park where you can see a lot from the car, or visit some of the sites quickly and in reasonable comfort, and still have time at the end of the day to head (an hour) into Palm Springs, where you can stay in some seriously cool lodgings, splash in the pool, drink Margaritas at sunset, head up the Aerial Tramway for sundowners, and all with a very cool mid-century vibe.

Then I'd head to the coast to Santa Barbara, bypassing LA for now, and follow Hwy 1 north from San Luis Obispo to Monterey, stopping at the Hearst Castle in San Simeon, at Big Sur, Point Lobos (state park with stunning coastal scenery and a lot of wildlife) and Carmel. On the way, you could hit some vineyards around Santa Barbara and the Santa Ynez Valley, of "Sidways" fame - a very happening wine district, less touristy and less expensive than Napa.

Then visit the Big Basin Redwoods near funky Santa Cruz and into SF for the rest of the trip as planned.

Even with two weeks you will be rushed with this itinerary, but nothing like trying to fight the heat, hugely long drives, and masses of humanity that your main proposed route would cover.

For beaches, during your stay in greater LA, you can stay in Huntington or Newport Beaches and still have easy access to Disney, or just before flying out, stay in one of the South Bay beach suburbs in LA, such as Manhattan or Hermosa Beach, both 20 min. from LAX.

And please, not the Mustang. I believe that if they weren't rented by Brits none of the rental car companies would even have any.

(And I hope you know that a big prevails throughout this post.)