2.3m is not outrageously low, but it is noticeably lower than typical victorian properties, and even pretty low for a new-build. If it's a choice between (for example) 2.3m on the first floor and 2.2m in the loft, or 2.4m on the first floor and 2.1m in the loft, I'd go for the latter. As LondonMix says, people will be more forgiving of lower ceilings in the loft when it comes to selling.