There's so much choice, unless you really dislike route planning you might be better off devising your own to appeal to your interests and style of cycling.
Through a large part between Devon and Scotland, the major choice is which side of the hills you go, Cotswolds, Peak District, Pennines.
West and staying in England is the shortest and fastest route, it's also the most urban and IMO the least scenic.
Staying West and going into Wales adds plenty of scenery and not really very much distance, you still have a fair bit of urban landscape to negotiate when you leave Wales behind.
Love hills? Go straight up the middle, see the best of the country and tick off some classic climbs. Or with a bit off careful planning, you could avoid the roads with the steepest gradients and still have plenty of scenery, have a look at the Pennine Cycle Way as an example.
Going East adds a bit of mileage and is probably the least common option, if you want an easier ride with very little urban to navigate and as flat as you'll find it's still worth considering.
I chose East, it was an additional 120 miles to the Western route I considered, which over the time wasn't a significant increase in the daily mileage. If I did it again, I'd be tempted to take a bit longer and go straight up the middle.
Scotland has a similar set of options - Island hopping, East, Central, West, though the pros and cons aren't so clear cut.
