I second the advice saying go for a light, showerproof jacket with layers underneath. This is a much lighter and flexible arrangement - easy to pack, easy to care for and if the insulation is provided by layers underneath, you can tailor it to suit the day's weather, even making adjustments throughout the day if weather or activities demand it.
As an 'acclimatised' Californian, you will probably find at least some of the days a bit cold. The UK as a whole must always be treated with respect weather-wise, due to its latitude and the fact that it has a maritime (read: fickle) climate. I lived there for the first 42 years of my life, so believe me, I know. You just have to know that most of the UK's weather, especially in the north, comes bowling in from the north Atlantic, then you have a handle on the weather.
The maximums you are likely to experience are in the 10 - 16C range, with exceptional days warmer or colder. Without wishing to put a damper on your plans, you may also experience days when you will suffer a combination of 10C, wind and rain. For sure, you will be glad you packed that jacket!
Much depends though on what you plan to do and where you plan to go. You already know, I'm sure, that weather is less important when visiting cities than when out and about on the beach, in the forest or on the mountains. I always factor this into my packing. Footwear especially must be chosen to suit your days. That said, even if you plan to get among the Highlands (recommended), you can see a lot from the comfort of a car, coach or through a tea shop window :-)
I hope you don't let this weather report get you down. I'm guessing that you've done your homework and know this already. All it takes to have an absolutely fantastic time in Scotland is (a) a little planning and (b) some fortitude. Believe it or not, once you are accustomed to the climate, walking in cold - even wet - weather can be lovely, especially if you're exerting yourself a bit, when the cooler temps are a godsend.
There are also some experiences that are actually made better by inclement weather. One of these might be lunch or dinner in a rural pub. Getting hunkered down in the warm confines of a traditional Scottish pub while the rain bashes down outside, a hot meal in front of you and a locally-brewed Scottish ale beside it, is one of life's pleasures, in my opinion.
Have a great time!