it depends on the venue, some will let you and others wont. And it depends what level. Generally in prelim unaffliated you cant compete if you have BDpoints, Novice is normally between 20-30 points and Elementary is normally something between 30-50 points.
I did an unaffliated once tho where you werent even allowed to be registered with BD, even if you had never competed the horse ridden in BD.
Agree with PG, the organisers usually make their rules of entry clear on their schedule and will decide how many points excludes you from each class. A lot do follow BD with no more than 37 for prelim, 74 for nov and so on but some dont so you need to make sure.
The only other thing - BD members cant compete in unaffiliated if the prize or prize in kinds exceeds the value of £25 (they have just revised it from £15)
That's really helpful, thanks. The horse in question has 35 BD points, but the rider isn't quite there with dressage just yet. But that's no problem really because there's some local stuff he can do to get him ready.