Don't know what dictated Harwich rather than Ipswich as the turning point for Norwich but.. Brentwood - Ipswich is a doddle. Nice line feature all the way with convenient way points at Chelmsford, Witham, and Colchester. I guess you could use these and keep to the right of them for Harwich. but it does put you close to D139 I think. I would choose Ipswich - great landmark in the Orwell bridge near to what was Ipswich airfield until the developers grabbed it for housing.
Ipswich to Norwich could be busy on a sunny day with lots of small airfields and aviators not having to talk to any authorities. Easy on workload other than the look out. Another great line feature in the A140 which goes all the way from the west of Ipswich up to Norwich. Taking Harwich Direct or Orwell bridge direct puts you to the East of this line feature and in quite open terrain with little to assist on a murky day. If following the road look out for Mendlesham mast at 1234 Ft (QNH) about ten miles north of Ipswich. You might want to talk to Wattisham but you miss the atz if you follow the road (just enter the MATZ stub). Beyond this there is a small chicken s**t powered power station on a disused airfield at Eye, just south of the town of Diss. By now you will want to be talking to Norwich who are only really interested in you from about Mendlesham on. Beyond Diss you pass Tibenham which is busy on a good convective day and you will be advised of 'numerous radar returns from what are believed to be gliders'. If you keep to the right of the road you should be clear of the intense activity and any winch work but all around here expect to find gliding activity if the conditions are right. You may alsi be passed information about traffic from Old Buckenham, about three miles North West of Tibenham - quite an active field. Following the road puts you mid way between Old Buckenham and Seething (my home field). You will find two old type masts at Stoke Holy Cross about five miles south of Norwich and on the direct track from Harwich or slightly east of track following the A140. Expect to be asked to report visual these masts or passing the Southern by pass for Norwich which is a part ring road around the south of the city. 27 is the expected runway in which case you will route towards the city then turn East to downwind. The airport looks to be North of the city on the half mil but is actually enclosed a little by development now. It should stand out reasonably well though with a cross runway directed at the terminal building and a longer main 27/09. Coming from this direction you are unlikely to mistake Coltishall for Norwich but if you are in the circuit with a couple of Jaguars you have gone too far!.
Norwich air traffic are generally friendly and will help you out if they know you are on QXC.
For the return you might want to follow the same route back or ring the changes and route to Felixstowe then Southend. Less to assist on this route but going South you can afford to head pretty much due South until you see Ipswich then keep it on your right. If ever in doubt go a little further left as the sea is a great line feature which will get you to Felixstowe. The only thing on this route is not to fly over Leiston (sizewell power station) I think they still shoot you down for this on the basis that you might be trying to use your C150 as a nuclear detonating device - only up to 2,000 ft I think but best not to test them at 2001 ft!.
So that is the journey - have a good one