What's the proper American English and British English word for the wind shield used on beaches?
Both my (English) parents and I refer to them as 'wind-breaks'. Windshield like its namesake on a car sounds American. In England cars have windscreens. Windshelter would be fine as that is exactly what you'd be doing on a British beach!
Portable Windscreen, as advertised at MarthaStewart.com.
Since Martha Stewart is a U.S. business woman with an eponymous U.S. business, I suspect this is what it's being called in the U.S.
Other terms at Amazon.com in the U.K. call it a windscreen, windbreak, windbreaker, and... windshield.