If you’re not 21 yet, then you’d need to get used to American hang ups about alcohol. But other than that factor in favor of Canada, I think Berkeley would carry the most academic credibility once you are back in the UK, with UCLA the runner up. Many people in the UK (especially those in their 40s and older) would consider Berkeley as on the same level as Harvard (partly because grad school reputation counts for more in UK/global rankings and partly because most people have heard of Berkeley).
However in terms of lifestyle, UCLA has better food, more of a fun reputation and the weather is better. And other than scenery (it’s hard to match the view of the Golden Gate anywhere in the world), LA probably has more to offer as a city compared to SF (we live near SF, my son chose UCLA over Berkeley).
Regarding diversity, anywhere in the US would be very ethnically mixed compared to the UK. But that would be particularly evident in California.