@Ruthsoph - ASOS (including Topshop), New Look, Uniqlo, River Island, H&M, Mango, Nike, Nobody’s Child, M&S, John Lewis all have maternity ranges. But probably best looking online.
I‘be been browsing and looking at Vinted too, but waiting until I start getting a bump and seeing what will last me from my normal wardrobe.
Ive bought a band for jeans from Jojo Maman Bebe that attaches to them. One friend just used that on normal jeans the whole way through pregnancy!