(By "britains capital", I'll assume you mean of the Anglo-Saxon part of Britain)
It would require a multi-PoD TL.
First, Mercia overlordship must be maintained. One way or another, Wessex should be integrated within Mercian domination (yes, I know it was dominated IOTL, but more deeply and of course maintained so).
Then, Thames form a natural trade way that runs deep on England (though, contrary to what Sakura_F said, London and Thame predominance on Atlantic trade wasn't established before the second part of Xth century)
You need to get rid of it, or at least make this importance relativised, but not only there. Ispwich was a clearly greater, more solar point than Norwich that was only a secondary point and not continuously before the late Anglo-Saxon period (when Thames town began to really impose their dominance).
Now, economical importance was partially due to royal influence : Atlantic emporiae grew thanks to royal support and development. (And it seems that Norwich, at least in the last period of AS England, was a royal settlement)
Unfortunatly, the "capital" of Mercians, Tamworth, is at the opposite side of England. You'd need an Mercian overlord king to move his dynastic holding or at the very last to create another one in East-Anglia long enough to devellop a Winchester-London like relationship (an emporia as capital being really unlikely in a first time).
You'll need then
1) Mercian overlordship for unifying in a sole entity at least 2/3 of England and preventing political dominance of southern Anglo-Saxon kingdoms
2) Radical displacement of Mercian political centers from west to eastern part of England (possible trough a son of Mercian king serving as sub-king, as it happened OTL, keeping his holding there after taking over Mercian rulership).
3) Great trade centers and emporiae as London and Ispwich being get rid of
4) Royal support in Norwich.