We used BOTH luggage transport AND backpacks because our trip continues onto Wales and then to London to visit friends and relatives.
Our packs contanied 2 liters of water each, plus 1 change of clothes (pants/shirt/socks), 1 down vest, 1 rain jacket, etc. Mine also included a small cook stove, fuel, coffee. Energy bars & energy gels. Plus insulin, syringes, back-up diabetes supplies for an insulin pump, lunch. Average pack weight was about 12#, mine was closer to 20#.
Frequently we encountered cold enough weather that we had shirts, fleece, down vest & rain jackets on, just to stay warm, so that lightened the pack weight a bit.
2 of us actually ran out of water on one of the days when we were in a more remote area. Surprisingly there are not a lot of places to stop to refill on the route. I was actually advised to take a SAYWER MINI or similar water filter to filter stream water but opted NOT to do so. I still think it was unnecessary, but it is necessary to make sure you have enough water for the day.