Hi Linda
I'm a fellow Californian (expat now). Get your passport at the post office. You can STAY here for six months without a visa. You should visit or call a British embassy before coming if you plan on staying longer. They won't tell you, but you CAN apply for permanent residency here, too.
As for accessibility, it really depends on exactly where you're moving to. London's bigger stores etc do have accessibilty, but most of the smaller shops do not. I don't believe they have the same laws here as in California.
If you have kids, you should make sure that this is really what you want, before moving here. There are a few problems with schools here (our public schools as you would know them), but some of them are great. Check out the area you plan on moving to and its catchment areas so that your kids don't have to put up with a crap school.
Be ready for an entirely different culture. People aren't as open and friendly here, especially not like California, although most are polite beyond belief. It takes a very long time to make friends. I imagine, though, joining a social club or special interst group helps.
Try searching google for disabled accessibility London, this may help.
I hope you enjoy your planning and plotting!
-Lili
