I have nerines in a pot and in the garden near London. Both lots are flourishing and the garden ones have the bulbs on the surface. I would say they seem pretty hardy. Can't help about the white variety. Mine are pink.

If you are talking about 'Sister Agnes', it is more tender than most. We occasionally get frost damage on it here in the subtropical desert. 'White Sands' is much hardier, if shorter, but I don't know about other white cultivars.