It might be worth having a quick chat with Essex Marina. It's more isolated than Fambridge but you can always cross the river to Burnham using the Ferry (burnhamferry.co.uk). It's also lot closer to the estuary.
Last time I saw him, Tomahawk kept his cat at Fambridge, and might be a useful contact.
If you are planning to keep the boat out there long term, it might be better to keep EU VAT status. After all there are more potential buyers in the EU, and if it's ideal for the SOF I suspect you'll probably get a better price for it out there than here.
I've used the public hard at Maldon Promenade Park to launch my dinghy in the past. Looking at the image below, its at the top, to the left of Maldon Yacht Club.
I contacted Crouch Harbour Authority last week in order to visit my boat which is moored mid-river at Fambridge. I've quoted their response below.
Cheers
Phil
Sounds like that is what he should be paying somebody to take it away!
Mind you, moor it out mid river it would be a good place to put your strategic stockpile of toilet rolls.
The OP wanted to charter a sail boat. If I remember correctly, to do that you need some sort of qualification, and a Day Skipper certificate meets the minimum requirement.
I think I'd do the Day Skipper theory over the winter months first. Then I'd book the Day Skipper practical afterwards as a holiday. You'd want to do it somewhere tidal, in other words not the Med, but I believe that courses run from Gibraltar count as tidal.