Populous built Allianz Field for Minnesota United - 20k capacity. Don't be surprised if it's similar to this:
The new home of Major League Soccer’s Minnesota United, Allianz Field boasts the latest in amenities and technology while also being sustainably-designed to minimize environmental impact and energy usage. Its elegant “flow” design concept features LED lighting throughout the facility. The...
populous.com
EDIT: When I say similar, assuming our project spend comes in at £100m as mooted, you'll sacrifice some of the luxury that Allianz Field has. That being said, the design as a template probably won't be dissimilar, though remove the top tier in their ground and remove the corners with a few additional rows.