Andrew Owen has released a small demo of a key component of the U3.5: Pax Britannia gameplay experience:

The music player is working well enough for a small demo. It’s not very polished, but it’s all 11 tunes running on the ColdSnap Audio engine. All you need to use it is a ZX Spectrum 128 emulator that supports the .SZX file format.

You can grab the download from this post on the Ultima Dragons Facebook page, and I would imagine it will find its way onto the Downloads page at the Pax Britannia subdomain at some point.