Just from a quick look at the map, how about nipping east to Boston and the Wash, then doing the coast (as near as you can on the roads) of East Anglia, and back up via Cambridge and Peterborough? Not sure of the mileage, but you could tweak it.
I happen to know there is a CTC route that does the North Norfolk coast, part of a bigger loop.
I reckon with so much training time, 90 miles a day is doable, yes. Will you be supported, or carrying all yur own gear? Camping or B and B? Those are the sort of things that can affect your mileage, as well as your fitness. Might be as well to build in some rest time - 90 miles a day for 4 days is one thing, but for a week, you might need a rest day in the middle.