You're only 41! At size 18, you're likely overweight, but unless you're very short you won't be obese. If you decide you want to diet and/or change your exercise regime, you certainly can. But it sounds to me that it's more the jeans and tops you're tired of.
I got to that point myself and now look for plain trousers and more creative shirts in nice fabrics. That's my basic uniform and having it means I can match new items with what I already have.
Have you checked out Zalando? It's a German company but still ships to the UK and covers import charges because clothing shops are so rubbish in the UK that it gets a lot of business from the UK! The shipping time is about a week but the quality is high and it's a reliable company. There are tips on putting outfits together and there are sales.
If you have money to spare that I don't, another good option is Sezane, a French company. It can be hard to get the sizing right because items are often very baggy or short. Shipping is expensive. However, there are some very creative and casual looks though that I think someone who prefers jeans and T-shirts might feel comfortable in as an upgrade.
I have also found some inspiration here recently:
www.wolfandbadger.com/uk/
I'd start by asking your friends and family or a friendly sales agent at a shop which colours and cuts suit you. I made mistakes when I was branching out from jeans. For example, I found that pale colours really don't suit me and neither do wide-legged trousers. If you're a size 18 and have shape, you may find they make your legs look too thick. A boot cut or straight leg flatters me much more. Flares make me look like I have chicken legs (skinny compared to everything else). It might be work dropping in at a John Lewis and parking yourself in a changing room with different styles of trousers or skirts or whatever and taking photos that you can examine at home before spending a lot of money.
Good luck.