Kent has a busy, well-organised equestrian scene with plenty of riding schools, Pony Club branches and professional yards, and a good supply of ponies that have already been produced and competed. For families in the south-east it is often the most practical county to search, with viewings reachable without an overnight stay.
Expect south-east prices. Land is expensive, keep costs are high, and that is reflected in what ponies sell for — a comparable pony will usually cost more here than in the west or north. What you gain is convenience and choice: more ponies advertised within an hour's drive, more chance to see a pony at a show before you commit, and more professional yards where a pony has been properly schooled. Grazing is often limited and many ponies are kept on livery, so ask how much turnout a pony is used to.