Having read the British Cycling rules for races, I couldn’t find anything to define what was or wasn’t a mountain bike for the purposes of an XC race either.
The subject seems oddly ‘political’ at times, a bit like the 3 Peaks Cyclocross being definitely not a ‘Mountain bike’ race -due to historical landowner concerns(?) about mountain bikes, although that is run under different rules to XCO.
In the case of an XC race, frame geometry and top tube angles don’t really seem that important in the grand scheme of things, especially as they are not defined anywhere -It’s not as if they offer an unfair advantage, disadvantage or safety concern either. If the course is relatively technical, then there probably is no advantage.
In this case, it looks as if I’ll be trying to spin fast on my chunky hardtail with its fat tyres and hoping that it isn’t too muddy.
I’m sure it will be fun anyway.