Most European grapes are grafted onto an American rootstock. The two reasons for doing so are to avoid infestation by a specific insect and to improve vigour. As yet the insect in question has not made it over to Britain, so the first reason is void. One can also argue that since the Romans grew grapes in Britain (and later medieval monks), the rootstock is not essential to providing a good yield. We see many grafted grapes in Britain because nearly all are imported. The grapevines I have are all Dutch.
As with all seed grown fruit, you could have a non-starter or you could have the next big named variety. Congratulations for growing one and I hope it's the latter.
