Britfag here. London to manchester sounds kinda shitty but I dont really know the route so I'm imagining it to be built up. I bet it would be really good and alot more tranquil than I'd probably of thought, although as already mentioned its a pretty populated area.
Also I think you could chose some more peaceful places overall to walk that wernt as built up. Have a look at the south west coast path (630miles) which runs from Poole harbour in dorset all along the dorset coast along the south devon coast to cornwall then along north cornwall and north Devon coasts.
http://www.southwestcoastpath.comI'm sure there must be something like this that runs around the coast of Wales too which you could also go off and explore a few mountains too. Also same thing in scotland.
