no, you can travel on any train, doesn't matter which operator

London to Norwich using open off-peak or anytime tickets always has two valid permitted routes, via Ipswich or via Cambridge/Ely, regardless of engineering works or anything else

boring step by step guide to checking these things for those who might need it in future:

find your journey in the routing guide (wouldn't recommend printing this) User Posted Link

from that we see the codes for Norwich to London Group are AB+WA, EA and KE

look the codes up at User Posted Link

and you'll see that via Cambridge and Ely are permitted routes (AB+WA and KE) as well as the direct route (EA)

Posted By: s10 yellow, Nov 29, 15:29:17