The edging we are recommending for this area of your project is
Premium 150mm Steel Edging Inset At Ground Level
Description
Our premium 3mm thick 150mm high mild steel edging has an additional bracket running the length of the steel to ensure a crisp, beautifully straight line - perfect for creating divisions between gravel, planting or a lawn.
Colour
This edging will arrive to site untreated and will rust and age over time which is the intended finish of this mild steel product.
Specification
300mm guide pegs to be hammered into position below finished surface level, so as to not be visible.