Yes I have just joined Essex Marina as this is as far as I know one of two reachable petrol outlets - there is one here - one in Gillingham and one at Bradwell I think I dont know all of the outlets and will need to know as I have just bought a motor cruiser with a petrol engine and wonder where I can go and how far I can go ...
Starting up North. Suffolk Yacht Harbour, Bradwell, Essex Marina, Gillingham, Medway Bridge Marina, Ramsgate, Dover, Eastbourne and Brighton are your options for alongside petrol.
Further north they are sparse, further west than Brighton and there are plenty of options in the Solent.