Can't say for the rest of the UK, but here in Glasgow it's WAY slower than in SoCal. Especially when you need to get things done - it take quite some time to get repairmen in, callsback from merchants (unless they want to sell you something), etc.
[This was prior to the Covid. For understandable reasons, it's exponentially worse right now.]
Summertime. We went for a walk yesterday to a shop to pick up something we had ordered. I was wearing jeans and a sweatshirt and was fine on the walk over, but it turned cooler, the breeze picked up and it started sprinkling an always-cold rain on the way back. I was definitely chilled by the time we got home. In July. We did not go out on Monday to pick up the item because it was raining sideways basically all day. Again. Be prepared for that, depending on where you settle. While I'm ok with the gray and rain, some people just hate that it rains so very much here. And then there is the dark in the wintertime. Dark at 4pm-9am. As opposed to high summer, when you can read by the ambient light at 10pm and it gets that light again at 4am. If you are disturbed by your room being too light when you are trying to sleep, you may want to plan to get black-out curtains.
One suggestion on "stuff" - if there is any chance you'll need or want a thing over here that you have at home (say, a particular coffee pot) and it can be made to work over here (with a voltage transformer, if it's electric), you might want to do a little online search to see what the replacement cost here would be. Things were more expensive to replace over here than I expected when we were jettisoning stuff to move over. Of course, you do need to factor in the cost of shipping it over. Also, if you plan to move it all home again at some time, it's more expensive to ship your household goods to the USA from the UK than it is to move to the UK from the USA.
The shops typically close at 6pm here, too - some a bit earlier. So you might as well get used to that. Some of the larger ones in the shopping centers may stay open to 8pm (?), but the "usual" shops shut down early here. Some do not open on Sundays, but most seem to be open then. I had heard before we moved over that there was one night a week when a lot of shops stayed open late, but I haven't found that to be the case where I am.
Good luck!