The edging we are recommending for this area of your project is

250mm Steel Edging Inset At Ground Level

Specification:

Constructed from 5mm thick 250mm high mild steel and held into position using 400mm metal spikes.

No colour finish.

Note that due to the nature of mild steel, these may arrive on site as bare metal which will rust in time.  This is the intended finish of the product and is a self protective coating.