I've been to Chatteris too many times already. But then I'm not riding on account of not having enough time to be fit enough in time etc.
I will NOT ride to York over the fens ever again.
You could do the Bridlington route then. No fens, but a bit flat just south of Bourne for about 10 miles. Plus the bit around Lincoln, but not as bad as the Fens. Lincolnshire and Yorkshire Wolds feature in this route.