Is it Listed?
See https://historicengland.org.uk/listing/the-list/advanced-search/ and enter ‘Milestone’ into ‘Asset Type’. Listed Building Consent by the Local Planning Authority would be involved, if changes were envisaged; the listing is intended to keep it in the original state as when it was listed but does not preclude alterations provided the necessary consent is gained. Repairs not affecting the character or appearance do not require consent, but alterations do. It’s a good idea to check with your local planning authority if in doubt. Authentic repairs, using the same materials (or as near as possible) would include painting if the original was painted. Painting something that had never been painted before would be an alteration requiring consent, which would be unlikely to be granted. Repairing broken stones by concealed stainless steel bolts or using glue would generally count as a repair, as would, stitching together broke cast iron plates. Total replacement of the stone or the metal plate would be an alteration, but if what was left at the end was merely a replica (which would not be listable), then strictly speaking consent for demolition of the original would be required (unless it had been demolished in a road traffic accident), or a request made to remove the list entry.
Should the milestone be Listed?
Listing provides a ‘snapshot’ of the milestone in place at a point of time. It is designed to protect our built heritage and can be a helpful tool to both deter and detect theft because the milestone can be provenanced. There is no obligation on the owner to look after a listed item but to ensure that it is not altered without appropriate permission. If the milestone is ‘at risk’ and you want to provide more protection, perhaps from a property developer, you can find guidance and application forms on https://historicengland.org.uk/listing. It is a relatively straightforward process but you will need to submit a fully detailed physical description, its history, clear photographs and accurate map references because the Historic England inspector will not make a site visit. And if you are planning to restore a non-Listed milestone, delay any listing application until after the work is completed.