Bear in mind that this is just a BT Retail customer help forum, so its not really the best place to post.
What part of the BT Group is the interview for?
Be yourself is the best advice 🙂
Your interview isn't to test what you know, It's to test how you think and how you have a conversation, you'll be asked questions that have no right or wrong answers, BT want to know how you make decisions and how you work with others, you'll also spend time sitting with advisors and have a tour, to make sure that BT is the place you want to work.
BT will put you through weeks and weeks of training, prior knowledge isn't required as you'll be taught everything for your role, even once your classroom training is completed and you hit the floor, you'll have support in place for weeks after.
Good luck, you've already completed the pre-assesment tests, which is the hardest part of the process.