Generally an assault is any action which causes another to "fear for their safety". In some cases a mere look can be adjudged assault, depending on circumstances. But it's that "circumstances" and what any witnesses report that would be the key to any hope of taking things further.
So it is illegal to threaten to throw something at someone ?
It can be illegal to just pretend to throw something with no words spoken, if it had the effect of making someone scared.
The classic finger-across-the-throat motion beloved of wannabee gangsters can be classed as an assault, again, depending on circumstances.
That said, there are other offences, such as threatening behaviour that might be more appropriate ..
www.cps.gov.uk/legal-guidance/public-order-offences-incorporating-charging-standard
is the fount of most knowledge.