"There’s a vertical copper water pipe directly behind which stops appliance sitting flush."

Among American models, there's no such thing as a dishwasher that shallow (20"). You're going to have to move the pipe out of the way.

Dishwashers are typically 24" wide. In order to actually work and to get water into all the right places, dishwashers have spinning spray arms. By their very nature, these arms turn in a circle. So the dishwasher's interior depth is approximately the same as its width.

In practice, you lose a little space for the baskets and for the door. So, the appliance is often a little deeper than wide.

There are 18" dishwashers, but they are so small, the manufacturer tries to extend the depth even if that doesn't result in optimal spray patterns. That's one of the reasons why they are not very popular. And of course, reducing with wasn't what you asked about.

In other words, there is very little a manufacturer can do to build a dishwasher that has a reduced depth. The physics imposes some hard design constraints. And even if they managed to shave off a few inches, there wouldn't be much of a market for it.

I agree and suggest you move the pipe. Ask a plumber to give you a quote. Frequently, moving a copper pipe isn't really a big deal and won't take the plumber very long to do.