Clipping cables to fences is generally regarded as not compliant with the regulations.
OK... I don't want to bury the cable. I'd prefer it exposed, above ground. What would the solution be? ... I consider my fence posts quite permanent and am happy that panels or even the posts falling over (?!) won't cause problems as the cable has enough slack and is very strongly secured prior to termination.
I think it's essentially a judgement call. To the best of my knowledge, the regs say nothing explicitly about clipping to fences. They say a lot about taking steps to minimise the risk of mechanical damage or stresses on the terminations, but leave the designer/installer to interpret that. In all but the most potentially traumatic of environments, to have SWA clipped to a wall would generally be regarded as acceptable - so it's really for you to decide how stable the fence/posts are and what would be the consequences of 'collapse'. From what you say, it sounds as if you are probably fairly confident that neither cable nor its terminations would be at appreciable risk if the fence collapsed.
Kind Regards, John