Hi
I live on the south coast and have been up to Broads Great Yarmouth about 5 times.
My prefered route from the Brighton area is:
London orbital car park; A12 to Ipswich; A14 for about 8 miles (sign posted Midlands, Bury (Norwich A140)) then A140 to Norwich.
The A140 is single lane for most of it's lenght but usually OK.
I use to spend a lot of time going backwards and forwards from Brighton to Thetford M11 etc but have always found this way to Norwich to be OK, takes about 4 - 4.5 hours