You can arrange an ad hoc demo with their UK rep. He has a fleet of demo bikes in Wiltshire. Chances are he’s got something similar to whichever bike you’re considering. Go to the Rose UK facebook page.
Root, instinct is that a 55.5 might be too small. That size fits me perfectly for off-road riding and I’m 5’10. I ride a 56 top tube road bike. Maybe you’d be ok but you’ll be right on the limit I reckon.
The 55.5 has a virtual top tube of 560 which is the same as my roadbike. I’ll know this time tomorrow whether it was the right call but i’m fairly confident that it is. I’m 5’10 btw.