Bradwell on Sea in the Blackwater Estuary is good in Northerlies, Well I say good, usually gusty ****ty wind but onshore.
About 30 mins from Southend, head towards the old nuclear power station.
Watch the shells at low tide and don't eat the mud as it's full of heavy metals (probably, allegedly) other than that it's great, It used to be a brill winter sailing place when the power station was running as the water there was about 5 degrees warmer.
I am off to sew an extra leg on the wetsuit i've just bought.