My 2004 Prius (which should have the same behavior as your 2005) definitely doesn't have consistent brake response: the first inch or so of travel is completely electronic and when the battery is completely full (having gone down a long hill) it doesn't do any regeneration at all and neither letting off the accelerator nor using that first inch of brake travel does absolutely nothing.

If you're in B mode, it will use compression braking (and if you're stopped, it'll bleed off energy by spinning the engine without any gas), but that doesn't change when you actually press on the brake pedal.

I rented a 2009 Prius once, and was surprised by the brake pedal being completely electronic. When the battery was completely full, I noticed the brake pump noise occurring immediately upon touching the brake, instead of partway down the travel like it usually does when regeneration is working.

My mom's Leaf also has a fully electronic pedal, but I've never driven it down a very long hill with a fully-charged battery, and there's no telltale brake pump noise, so I don't know if it changes its behavior with the battery filled to capacity.

Interesting, I've noticed my 2008 model Prius effectively using 'B' mode when the battery is full and I'm heading down a hill even thought it is in 'D' mode.

Maybe I just haven't noticed the change in brake feel or it is a UK specific feature.

The Prius can dump the energy into turning the engine over, just as a regular ICE would.

You can hear this by starting at the top of a hill with a full battery and in `D` mode. Then, as you roll down it, you can hear a noise from the engine as it is being turned over as it uses the regen energy.