I recently visited The Boardroom near Chester (amazing wall! well worth a trip) and noticed they have letters on their lower grade rope routes (4s & 5s). I found it slightly odd as most (all?) walls I've been to in the UK have no letter on these low grades.
Looking at the grade comparison charts etc, French/Sport grades have the letters on the lower grades (i.e. 4abc(+), 5abc(+), 6abc(+), 7abc(+) etc). The only thing that seems to match the system I've seen in most walls (at least around Greater Manchester) is font bouldering grades (i.e. 4(+), 5(+), 6abc(+), 7abc(+)). But those roped routes can't be using bouldering grades, surely? Do UK climbing walls use sport grades but omit the letter on grades lower than 6a? Or are they a different system entirely? I'm pretty sure I've always heard them called "sport" grades. The Boardroom have confirmed they use sport grades on Twitter.