You will not need to apply for planning permission to take down a fence, wall,or gate, or to alter, maintain or improve an existing fence, wall or gate (no matter how high) if you don't increase its height. In a conservation area, however, you might need conservation area consent to take down a fence, wall or gate.
You do not need planning permission for hedges as such, though if a planning condition or a covenant restricts planting (for example, on "open plan" estates, or where a driver's sight line could be blocked) you may need planning permission and/or other consent.