Can you damage an LCD screen with a camera flash, high-powered flashgun/speedlight?
A common camera flash/speedlight should not emit very much UV at all, unless you modify the device. UV will be blocked quite efficiently by most plastic materials (such as the ones used in camera flash "lenses")

See example transmission spectra below:


Source: https://www.gsoptics.com/transmission-curves/



Source: https://www.hitachi-hightech.com/products/images/8414/uh4150_data1_e.pdf