There are quite a few greyhound stadia that are not in the 'top echelons' of the greyhound world that don't have anything else going on. eg. the new Oxford Stadium leaseholder has interest in other dog stadia! Not sure whether they are suited to adding a speedway track/pits etc or whether they have hosted speedway previously?
I know that Hove in Sussex have a greyhound stadium but that may be deemed too close to Arlington?!