I don�t see why a 2-litre shouldn�t do 35mpg on that sort of run with that load. It depends on whether you cruise at 85mph or 70mph, when you�re able to, that is. Those 50mph - limit motorway stretches will improve economy but lengthen the journey. I�ve got a 1.6 Escort which does low-twenties to the gallon around town (these are mostly 2 or 3 mile journeys, mind you).
I drove it down to Kent and back twice recently, about 280 miles each way. The first time it did an amazing 44mpg, the second time a bit less (it was loaded up a bit more), but still over 40mpg.
The last trip took about 5 � hours both ways. There were delays on the M25 and at Junctions 6 � 10 on the M1 which cost me about 45 minutes. I only stopped once each time ... if you stop for coffee/snacks/petrol/toilet 3 times that will obviously add quite a bit!
The other thing is ... if you can travel on a Sunday there are far fewer trucks on the road, which helps.
I agree, it is tedious driving that far even if traffic conditions are good!