There is no sense in withdrawing your other apps, even if you feel fairly sure at the moment that you will Firm this Unconditional offer.
Wait for the rest of your replies, go to some offer holder open days, (even if just to confirm your current feelings), and then decide.
There's no rush, you have months to decide, (UCAS will tell you your reply deadline when you all your Uni's have made a decision).
As above, you can always give the Uni a call to check if there's anything else you need to consider about their offer.