Lots to choose from, what time of the year are you planning for? In the North of England the Coast to Coast from St Bees to the North Sea, across the Lake District spring to my mind, or the Cumbria Way, starting at Carlyle and finish in the south of the Lake District. In the middle of the UK the Cotswold Way, Offas Dyke and Glynder Way possibly? In the south the North Downs Way, High Weald Trail and the Sussex Border Path might be a good choice?
My suggestion is to go to
www.nationaltrail.co.uk for all of our long distant national trails, or visit
www.gps-routes.co.uk, many others displayed here, a lot of county walks identified, some long, others not so.
Last but not least, log into
www.bingmaps.com/maps and the UK Ordance Survey maps are displayed completely free.