So should open cockpit vintage gliders come with a
health warning then?

No, the gliders with canopies should have the following warning:

WARNING: Do not depend on this canopy for protection against the sun. Even
though you might not receive a sunburn through this canopy, you will still
be exposed to the sun's UVA wavelengths. The UVA wavelengths are a proven
carcinogen. While using this canopy you should still protect yourself with
opaque clothing and broad spectrum sunscreens.