There is the Beachcomber Campsite – www.lindisfarne.org.uk/beachcomber – near Goswick to the south of Berwick. It’s slap beside the Coast & Castles route. I passed it on the way north and it seemed ok but I can’t vouch for the facilities there. A (small) mix of tents, caravans and motor homes were using it. Just west of Berwick is Ord House Country Park – http://www.ordhouse.co.uk/ Further south, I can recommend Dunstan Hill Camping & Caraven Site about 1km south of Embleton, good facilities and with tents well away from caravans etc., but a bit pricey at £9. Enjoy your trip!
There speaks a man who knows the value the money! I will investiagte all three options – thanks Edvardus!