Bra help

So I’m 15 and quite skinny but I have pretty big boobs for my size. I’m a 30 DD (UK) and I’m really struggling to find bras that fit.

Any bras that I’ve seen that are my size are not built for my shape and size. As in the shoulder straps are really long and not very adjustable. They also look like bras that my grandmother would wear and that doesn’t make me feel confident with my boobs.

I’m just looking for is a plain but nice looking bra that is suitable for my size and age range. Are there any shops in the UK that sell bras like these?