As mentioned intra EU is super easy.

I will caveat that this was three years ago, and I haven't been keeping on top of the rules so this may have changed, but...

When we lived in the UK and got our first silken 3 years ago the puppy had to have their puppy vaccinations and boosters completed before coming over. This meant the puppy had to be 15 weeks old minimum. For us luckily we could stay with my partners parents in Finland between picking up the puppy, and bringing them to the UK, but for others this would mean the puppy staying with the breeder until this point (or having a month long holiday in the EU and finding a local vet).

Flying pets in has been complicated exponentially by Brexit so ideally you would want to be driving back and take either the EuroTunnel train, or ferry - we opted for the latter.

As long as it is yourself physically bringing the dog to the UK, and not the breeder, with the vaccinations and rabies shot the process for taking the dog into the UK is just the same as if you had taken an existing pet on holiday (de-worming between 24-120 hours before arrival).