Phil, one of the nice things about this trip down the coast is that there is lots of info on it. Its been over 20 yrs since I did it, but even then this book was around,
Bicycling the Pacific Coast: A Complete Route Guide, Canada to Mexico: Vicky Spring, Tom Kirkendall: 9780898869545: Books - Amazon.ca
which I used and was handy for knowing distances between campgrounds, towns with facilities etc. This was pre internet of course, but its always handy to have a good idea of what is where, and especially to plan where to sleep state park wise. I'm fairly certain the distances between campgrounds was never much more than about 100km or 60 miles.(ish)
re weather, having a long sleeved thin wool or polypro top, plus bike shirts, plus a fleece and a rain jacket/shell, will pretty much cover the range of temps you'll encounter. I may have taken a thin winter hat but I dont remember, those "buff" things are pretty versatile also, but as I said, do some internet searching and you'll easily find lows and highs for that time of year in various spots along the route.
Having a few days in San Fran will be fun. Sure, its a touristy city, but I spent a bunch of days just wandering around and it is a neat city, so have fun. You will notice that there are lots of panhandlers, people asking for money, the climate of california means there are more homeless people in general, so just be aware and being from London, just use your common sense of being aware of your surroundings.
Enjoy Vancouver as well with your relative there, I'm sure this whole trip will be a really neat experience.
riding down the coast is a big tourism draw, so my comment on having a mirror is especially important on a road where people renting cars or driving those ridiculously huge American recreational vehicles will be driving along looking at the sights--having a few seconds of heads up of what is behind you (especially with an oncoming truck for example) on a narrow stretch is very much a life saver.
Do think of a mirror type that can easily switch to the other side of your helmet, or handlebars, than you are used to.
I use this, and it can be found for about 18 quid online , here is a good review of it by a couple who biked around the world:
Take-A-Look Mirror | TravellingTwo: Bicycle Touring Around The World
cheers