What tips or advice would you give to someone interviewing at Nationwide Building Society?

Bare in mind that this is a position which requires you to sell.

Prepare for a practical task - to ‘serve’ a customer with some query (eg how to transfer £10,000 using CHAPS payment and how much fee would be, how long would it take to process the payment etc). You’ll have 10 min to prepare and 1-2 min to answer to main task question and all supplementary ones.
Interview questions is all about customer service - don’t forget it’s not a bank but a building society (largest in the world, 16 million members in UK), they call their customers ‘members’ and try to create particularly friendly and ‘homey’ atmosphere in branch. Stuff mostly promulgated within Nationwide staff is ‘Future of Legendary Service’ and that includes 5 steps ‘Safe hands’, ‘Yes we can’, ‘Digital first’, ‘Safe handoff’ and ‘Careful closure’. Very likely you’ll be assessed against all these values.

Read up about their core values

Make sure you prepare for competency based interview questions

Just be yourself and try to relax and enjoy the time there.