I studied abroad in England, so I have a bit of a comparison between the countries. I'd agree San Francisco is very nice, as are many places along the coast of California. We have it all - Lake Tahoe, giant Redwood trees and Yosemite National Park (gotta see it sometimes), along with the capitol of Sacramento (where I live), and Monterey, Santa Cruz areas, which are vegan heaven.
Also:
Will I be able to buy cups of tea? Is it true that you all drink coffee and tea is hard to find?
Don't be scared...You can buy tea in any food market, and many places carry English teas. Starbucks and other coffee houses also serve tea. Some places even leave the soymilk out for us, but if not, just ask for it.
Will I be able to find chips, or do you all only eat those weird skinny 'french fry' things?
Any American chain restaurant has chips/fries. I wouldn't worry about it.
What is 'granola' and is it vegan? (People always seem to eat it for breakfast in films.)
It's often rolled oats, nuts, raisins mixed with honey or sweetener and baked. Check the box if you aren't sure, or choose a different breakfast cereal.
Is it possible to find ale, or is it just lager everywhere?
As others have said, you shouldn't have a problem finding all kinds of beer. Most grocery stores carry a wide variety of adult beverages

Anything you want to warn me about?
Public transit isn't as nice as in England, and it's harder to find in more rural areas. If you go to California or New York, choose Spring or Fall. If you go in the summer, be prepared for HEAT.
