Ok... obviously the term rash vest wasn't the right one
I have two ... One is a short sleeved top, silverskin lining and kept me warm after an hour in 21 degrees. Maybe 1mm neo, i don't know for sure, and I can't find the spec on the internet.
The other is a ralf tech vest which is quite thick, spandex bonded to neoprene, marked 3/2.
Both cost (and are worth!) a damn sight more than £2.99. i think they were both in the region of £35