26.8 was a common French size for 531, it means they grabbed a metric tube. I doubt they would put a seat tube in upside-down. Mercian is a high-volume custom shop, a lot like Waterford. Only the price on a Mercian is quite a bit less. I am not a big fan, but people seem to like them. I have never heard anything bad about them.
I don't really know the history of Audax in the UK. I didn't get the impression that there were all that many UK riders at PBP considering how easy it is to get there.