Not from the south, but this is what I think about the British (London) weather, at least compared to the (varied) climates in the north east US. It might not seem as helpful to a southerner, but it may be. Plus, it will give others an idea:
Rain: It rains more often but with less severity. A lot of it is slightly heavier than drizzle. It rains more days but for shorter periods, IMO. You will get stretches where the whole day is rainy, but then you might go a while where it's sunny. However, it could rain for a bit any day.
Cold: It's not as cold here as in the north of US. Far warmer in the late autumn to mid-spring, but it is so much damper. It always feels colder until that week or two where it starts to dip well below freezing. The heating situation may not be what people in the US who experience very cold winters (north, midwest) are used to. It might be more in line with what you get in the south where heating isn't vital, but more used for comfort on chilly days rather than to keep you from, you know, dying. If you are used to putting on a jumper rather than putting on a jumper and switching on the heat, you might not notice that much difference if you live in the part of the south where there is a noticeable but relatively mild cold season. Much of the winter, at least in London, seems to be in the mid-40s F with stretches in the 50s and dips around and below freezing.
Spring: Spring is brilliant and lovely. I've never lived in a place where the coming of Spring is more welcome (and I've lived in Scandinavia). And British Springs, at least in the SE, really live up to the expectation.
Summer: Short but can be lovely. Once in a while we will get really hot weather which no one is prepared to deal with. Shops don't tend to be air-conditioned, which isn't really a bad thing, but they sometimes aren't equipped to naturally move hot air out. Like, not having air-conditioning is great for the environment, but when a shop has a solid, glass faced front with no side ventilation, opening the front and the back (if you're lucky enough to have a back door) isn't really going to help. Being encased in a layer of sweat is what I expect if I go out on hot days here.
But, if you are used to southern weather, it might not bother you. When it is hot, it's not exceptionally humid like we get in the NE and the SE gets, at least in my experience.
Autumn: This is the saddest season here. I love Autumn, and having grown up in a place where people drive hundreds of miles to get to to see "peak" leaves during the Autumn, I am usually disappointed by it here. I don't know if there is better views in other areas of the UK. That said, I actually find it milder for much of the Autumn here.
Length of days: You are going to have to get used to it getting darker a lot earlier/staying dark later in the winter, and it being light earlier/longer in the summer. The latter actually bothers me more.