Matt,
IMHO, if you buy an OS map of Essex, virtually all of the byways are open and good to ride. An OS map will tell you where the routes are.
On
your bike, some will be easier than others.
Suffolk has loads of restrictions in place, so you will need to know where they are before setting out to ride a lane and as such an OS map should be taken as an "indication" only.
As for riding techniques, my only advice would be, stand up as often as you can and accept that there is no substitute for time in the saddle.
Set your tyres at about 15psi.
You will find that almost everone has an opinion in relation to trail riding, so I,m sure there will be other replies offering different advice.
Try not to ride alone and keep an eye open for the Hertfordshire TRF rides into Essex and tag along if you want.
Oh yes, and welcome