This is something you really should check with Mercian before you recieve the bike. When you have chosen your hubs they will spread the rear chainsays to make it fit but if you leave it then you will have to do it yourself or pay to have it done.
I had a custom-built Vincetori and had to spread the stays to accomodate a 9 speed block when I updated but it's better to let Mercian do it if possible.