Thanks for your inquiry, bra sizing can be a little tricky online. Brands we carry like Elomi are sold in the UK cup sizes with the US D cup equivalent next to it in parentheses. If you've previously worn 42J(7D) in US sizing, this would equal a 42GG UK. A 42K(8D) US equals a 42H in UK cup sizing. (Sorry I know it's a little confusing!)
Since this particular Elomi style is wireless, the center will not lay flat regardless of size, but that doesn't mean this couldn't be a nice style for you to try. The largest this bra goes up to in a 42 band is a 42G UK/42I (6D) US, which might be a little small since you've been wearing 7D or 8D cup sizes. If you were thinking of this more as a comfort style without the wire, you could try going up a band size to a 44G to give you a little more room in the cup.
I've linked a few more bra styles that might work for you. If you're having problems with a wired bra giving you good separation, especially in the center, I would try ordering up a cup size or two. One of the following size options: 42H UK=42K(8D) US, 42HH UK=42L(9D) US, or 42J UK=42M(10D) US.
I hope all this makes sense but if you have further questions I'd be happy to help you here or would recommend calling one of stores to speak to our certified bra fitters in the lingerie department. If you can make it in to a store, they also can help you determine the correct size and get you the right styles and fit you're looking for.