Traditionally, potatoes were planted a week after the usual last frost in your area - that varies from the end of March in Cornwall to mid-late May in NW Scotland.
A frost will blacken and rot the leaves and stems above ground; that means the tubers underground will not be able to grow and multiply.
Being buried a few inches deep will sometimes protect tubers from frost if they are dormant. That's how 'rogue' potatoes pop up in places where potatoes used to be grown.