There are approximately 1,600 IBA members in the UK and around half of those are currently "active" by which I mean have ridden a certified ride, participated in a rally or RTE within the last three years. Given your context, you might also mention that most IBA members are qualified advanced riders although I can't provide stats to support that. You might want to think of presenting advanced riding as being the starting point of IBA riding or the other way round: IBA riding is the natural extension of IAM riding.