I’d suggest you’re very unlikely to get a tester of such a niche bike, although i seem to recall Two Wheels cycles in the midlands got one in for somebody (but he was a very good customer of theirs). I bought a Salsa Big Mama and just had to bite the bullet, no testers anywhere.
Your best bet would be to find somebody off here who may have one and ask to have a go. There is one on here at least but that’s the XXL size.
Chevin in Leeds may well have a Niner full suss for you to try, other than them 29ers are still pretty niche to get a test on, and full suss even more so…..