1 Is it overgrown with weeds/ivy/hedge?
As long as it is safe to work on the milestone, this is a straightforward gardening job to make the milestone visible again.
You may want to consider preventing the weeds encroaching again, by using a weed-proof membrane from your local garden centre spread on the ground close to the milestone, held in place with gravel, perhaps. If you want to treat the surrounding area for weed control, 'Foamstream' is more environmentally acceptable than glyphosate (eg Roundup)
2 Is it covered in moss or lichen?
As long as it is safe to work on the milestone, it is usually a simple job to carefully remove moss with an old trowel or similar garden implement, without damaging the milestone.
Lichen can be removed with a bucket of clean soapy (warm) water and a scrubbing brush. Carry the water in a screw top container of five or ten litres capacity.
Like the weeds, the moss and lichen will regrow with time, but that’s life.
Do not use chemicals to kill off the moss or damage rare lichen growth.
3 Is it covered in dirt from the road?
As long as it is safe to work on the milestone, road grime can be removed with a bucket of clean (warm) water, Fairy Liquid or similar and a scrubbing brush. Carry the water in a screw top container of five or ten litres capacity.
If the milestone is very close to the edge of a busy road, it may become dirty again very quickly. If it is that close to the edge of a busy road, it is probably too dangerous for you to work on. You really should talk to that right person in authority.