OK lets's look at the sensible way to plan a trip to Cherbourg or the Channel Islands!
Assuming a Solent (or thereabouts - Carlo, can you confirm your planned depature point?), then it makes sense to top-up the tank at the last quayside fuelling point, so if you're heading out past the Needles, pop in to Yarmouth or Lymington to fill up. If going the other way, pop in to Portsmouth.
You now have a full fuel tank to start your journey with a more than adequate reserve. If your destination is Cherbourg, I would be tempted to avoid filling up there, and maybe take your own jerrycans, there are rumours that the fuel there isn't great quality. Worth asking people who've used the fuel facilities there. If your destination is Alderney, phone Mainbrayce Chandlers and let them know that you are coming. Plan to arrive before high water so that you can enter the inner harbour and use Mainbrayce's fuel filling device. You will need to carry the fuel from the petrol station, it's less than half a mile away. If you want any more information about re-fuelling at Alderney, drop me a PM. If your destination is Guernsey, you will have enough fuel, but plan to have the option to divert to Alderney.
It's easy and 150 litres is more than enough if you plan sensibly.