Should I seal sandstone? Based in UK
It is a personal preference to seal or not to seal. I have sand stone night stands. When we purchased I sealed incase I knock a cup of coffee over or spill something that would stain the surface I don’t want it looking bad. Yes things have spilled and they still look good. 
Another possible reason for sealing outside work is if it freezes hard. At my current location I won’t use a concrete finishing method I used in the San Francisco Bay Area , it worked great and looked cool there but the occasional hard freeze here would cause the surface to fracture, I believe the same may be true for sand stone but am not sure so if I used sand stone outside I would want to water seal it to prevent freezing damage. But as I said it is personal preference since sand stone is soft as an outside wear surface you may need to seal more often.