Have a look on cycle.travel for route planning in the UK and Ireland. Cycling UK’s forums contain plenty of touring folk with opinions on routes around both countries.

For the last part of your ride across Dorset to Poole for the ferry to the continent, I suggest taking the rail trail from Sturminster Newton to Blandford Forum, then the old lane by the river Stour via Shapwick to Wimborne Minster, then head south over the bridge to Merley and pick up the rail trail and bike paths from there south into the centre of Poole and across the lifting bridge to Hamworthy where the ferry terminal is.

If you have any extra time spare before your ferry I also recommend exploring the Purbecks to the south of Poole.