On page 15 on 'I sailed into a windfarm' there is a suggestion that Foulgers Gat is shut.

I queried this on the East Coast Forum and received this advice from Roger Gaspar, author of Crossing the Thames Estuary

Foulger's Gat is not exactly shut. They have lifted the buoys of course but then marked it with an avenue of turbines. If they are working adjacent to the Gat then the exclusion zone that accompanies such work does shut it off - definitely. Bear in mind that there is a lot of diving work going on putting cables in so you might not see something and there is a lot of sundry cables and anchors around the Gat not necessarily related to the work schedule. Always call the Mary Ann 1 - the Guard Boat and they will advise.

I also got further information from Dick Holness one of the authors of East Coast Pilot

Just to add/reinforce Roger's comment - Foulgers really is not shut, and the developers have no right to tell you that it is. It is possible they could shoo you away on the day if there's specific work going on causing their 500m exclusion zone to effectively shut the route.
There's a feature on windfarms in the latest YM where a sailor is quoted as saying Foulgers is shut. That is simply wrong.
If you can't get through on the day, then as the OP has highlighted, Fisherman's is not far away anyway - so long as it's not the day that they are laying an export cable across it! No known dates for the remaining two of these yet..
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Both of them know their stuff and although I've got no connection, I can thoroughly receommend both publications