H&M has a great maternity section both online and in some stores!
With my first pregnancy probably around 3/4 months but with my second pregnancy I wore maternity clothes from the moment I found out as I felt so bloated all the time and everything felt tight. I love jojo Maman (Vinted is a good place for second hand as it’s quite pricey) H&M, Asda, Sainsbury’s are all good places for maternity clothes. Plus oversized tops/jumpers and leggings in the next size or 2 up. I also got a few bits from SHEIN but it’s hit or miss
Try boohoo and PLT they have some lovely outfits x