Essex is a genuinely varied county for ponies — flat, easy-hacking country across much of it, a strong traditional horse fair and dealing culture in parts (Essex has long had an active horse trading tradition, including established fairs), and a busy Pony Club and riding club scene serving its large population. Prices near the London border in the west are high; further east toward the coast, land and keep costs ease off considerably.
Because of that dealing tradition, it is worth being a little more diligent here than in a purely hobbyist county — ask for full history, a vetting, and be cautious of a pony that has changed hands very recently without a clear reason. That said, the majority of sellers are ordinary families and small yards, and a good, honestly represented pony is just as findable in Essex as anywhere else.