By 18C, you will find people are wearing strappy tops and sandals and taking as many clothes off as possible... :)
(sorry, that's a bit of a stereotype. It's not really that bad. But 18 is a nice temperature here - and it will quite likely be warmer than that. Many days up in the mid-20s, which feels pleasantly warm, and some up to 30 or early 30s, which is getting too hot to me!).
Houses here don't usually have air conditioning, just windows (with no bug screens often - I got an expanding-width one in North America once when I was visiting and brought it over here, and it's been very useful). So you don't want it too hot really. Shops etc are air conditioned though.
Jeans, t-shirts, possibly jumpers, closed-toes shoes are fine for April/May/Sept, but for June/July/Aug, you can wear short-sleeved tops, perhaps shorts or skirts, sandals, etc. Always useful to have a cardigan or hoody to hand though. Be prepared for rain, though. Rain jackets and umbrellas and wellies (rubber boots), but you can get that cheaply here. It doesn't actually rain here as much as in the rest of the country, but it still might be more than you're used to. And then some summers have been fine and dry.
Online food shopping is much, much more common here than in North America, a very normal and standard thing to do - so just get that set up as soon as you arrive for all the bigger items, and then pick up smaller stuff regularly in the little supermarkets or the markets.
Have a look at the highway code perhaps, or even take a driving lesson here just to get used to e.g., roundabouts etc, and then you should be able to sign up for Zipcar, as long as you've driven for at least a year elsewhere, I think it is. That would increase your ability to get out and see the rest of the country. And investigate railcards as well, if you are going to travel often to London or further afield.