How do you guys plan your navigation for trips of several days?

Last year I cycled down the west coast of Sweden and across into Denmark using just my mobile phone with offline maps and a solar charger. But not having a place to mount the phone on the handlebars meant I kept having to stop every so often to check if I was on-track.

What tips have you got for pre-planning routes and following the plan on the journey? Are there any decent handlebar mountable cycling GPS devices you would recommend?

The journey I'm planning is from Sweden to England in early June. Starting in Gothenburg, Sweden, I am aiming to take a ferry to Denmark, cross to the other side of the country and get the ferry to Harwich in England and cycle to Wolverhampton and am trying to plan the best and most realistic safe route. I think it might be possible to go to Cambridge on the first day, Rugby on the 2nd and arrive in Wolverhampton on the 3rd, or is that over-optimistic?