Garmin 1000 or 800. 4 years with mine and garmin Dakota 650. They are the best, mines even survived submerging on winter MTB over Exmoor 🙂 and through Trans Portugal. Buy OS map UK and you will have all you need for turn by turn routing for LEJOG. It comes on micro sd card so you can swap it to new other devices in the future. Same if you go abroad, just buy the relevant country SD card. Download basecamp their free mapping tool software its best Ive used as a long term abuser on off road.
No mapping software or device can give you turn by turn off road.
You will need to recharge the Garmin 1000 or 800 every 10 – 12 hours. If you want longer batter life then go for Dakota 650 types as the use AA batteries which can be more useful if your on a long unsupported adventure. Alternatively buy a power monkey that will recharge the garmin for at least 10 days of use.