Cant you just put a few heavy concrete flower tubs in the way. I don't think you would need planning permission if the railings are bolted to the concrete as opposed to concreting them in, I could make them and install them myself if you were local!! If the railings are bolted rather than concreted they would be seen as temporary rather than permanent therefore planning permission would not be required, cheaper that way as well and can easily be removed should the need to be.