ornithogalum thtrsoides are these plants hardy?

I'd call them borderline hardy. They are native to South Africa and grow well in Western Australia. One US website says they are hardy down to zone 9 - which is quivalent to our climate in Cornwall. I'd expect to lose some, if not all, in a hard winter.

Mix extra sand and grit into the soil, and plant them a little deeper than it says on the package. A temporary much in winter could also help.