Popham - Alderney = 97nm (Just took me about 90 seconds on SkyDemon Light)
PA28=roughly 1 hour = roughly 10 US gallons
Add a worst case 60kt headwind at altitude en route = say 20 gallons total planned.
Plus, surprise, surprise fog or V low cloud rolling in over the Channel Islands as I arrive
and I would be very pleased I left with full tanks.
SD