UV umbrellas should be woven tighter to block out more sunlight, but even a non-UV umbrella will block a significant amount of sunlight.

I bought one of those UV-C detection cards that also detect UV-A and UV-B. Both a UV umbrella and a non-UV umbrella blunted the midday sun down from very high levels to low/medium levels.

I also bought a Japanese umbrella (brand Wpc) that is rated for UV and it blocks the UV levels down to zero.