I feel that planning such a route should be part of the adventure. Start off with a very rough idea and see where you get to by the days end. Follow your wheel and see where it takes you. Online route planning should be avoided. Take time to study an old map over a few pints of ale after a day in the saddle (a Brooks saddle at that). There could be for example, a small hamlet at the head of a backwater valley, with steep sided woods on each side, the vista opening up to an old church, honey coloured stone cottages, a village green and a small Inn. All details that an experienced map bore can spot when route planning that a computor cannot. Such places are the fabric of this country and it would be a shame to miss out on them for the sake of taking on a challenge.