So, look for good weight (in wheels and tires). Aero is important (especially riding west for 210 miles). There are a lot of ways to be aero – and wheels are the smallest bang for your buck – and hard to do with gravel tires. Any (non bike item) that touches your body is likely to have a similar impact. You are going to be glad the weight is light when you get to the second half of the C2C.
As for bikes – the Aluminum Canyon Grail would be great for this race – and is a good bang for the buck. But bike vs wheels – that’s up to you.